Modern Concept Science and Technology 10

246 LigHt Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 247Critical AngleWhen light travels from a denser medium to a rarer medium (e.g. glass to air as shown in figure), it bends away from the normal. At this condition, the angle of refraction (r) becomes greater than the angle of incidence (i). If the angle of incidence in denser medium is increased then the corresponding angle of refraction also increases. At some stage, the angle of refraction becomes 900. At this condition, the angle of incident is also called critical angle. Thus, the angle of incidence in a denser medium for which the corresponding angle of refraction in the rarer medium is 90° is called the critical angle. It is denoted by C. In the given figure (ii) angle of incidence is equal to critical angle (i.e. i = C).cri i i rr = 900 Rarer Rarer RarerDenser Denser DenserCritical angle and total internal reflectionFig (i) Fig (ii) Fig (iii)FACTS WITH REASONSCritical angle of glass is 42°. What does it mean?Critical angle of glass is 42°. It means that, when light passes from glass to air, at the incident angle 42° the corresponding angle of refraction in air will be 90°.Critical angles for some substances with respect to airSubstances Critical angle Substances Critical angleIce 50° Turpentine 43°Water 49° Glycerine 43°Alcohol 48° Glass 42°Paraffin 44° Diamond 24°Total Internal Reflection (TIR)The phenomenon of light returning into the same medium when it passes from an optically denser to an optically rarer medium with an angle of incidence greater than the critical angle (i > C) is called the total internal reflection.i rRarerDenserTotal internalreflection

248 LigHt Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 249ACTIVITY 6To demonstrate the total internal reflection of light1. Immerse an empty test tube in a beaker containing water as shown in the given figure.2. Gradually tilt the test tube and look at it from outside the beaker.3. At a certain angle of incidence on the water-air interface, the test tube shines brightly. Explain: The test tube shines due to the total internal reflection. The small thickness of the glass wall of the test tube in between the air-water interface is neglected in this case.Conditions for Total Internal ReflectionIn order to have total internal reflection, two conditions must be fulfilled. They are:i) Light must travel from a denser medium to a rarer medium.ii) The angle of incidence in the denser medium should be greater than the critical angle.Effects of Total Internal ReflectionTotal Reflecting PrismA total reflecting prism is a right-angled isosceles prism. It rotates light through 90° or 180° based on the total internal reflection. It is used to obtain a correct image of objects. Uses of total reflecting prismThe ability of total internal reflection by a prism is used to make optical devices such as periscopes, binoculars, microscopes, single-lens reflex cameras etc. The prism does not absorb or refract or reflect any rays of light. Every ray of light suffers total internal reflection and travels to another point. So it makes a clear image. Prism also helps to convert inverted images into erect ones so that it will be easy to use binoculars.RQP450450450450450Q RP450450900450450PQ900450R 450 450 450Inverted imageErect imageEffects of total internal reflectionTotal internal reflection through waterThe water is a denser medium compared to water. Its critical angle is 49° and its refractive index is 1.33. If we take a transparent beaker, fill water in it and use laser light to make an incident ray inside the water at an angle smaller than 49°, the red ray of light will refract. If we incident the red ray of light at an angle of 49°, it will be parallel to the surface of the water and perpendicular to the normal. If we increase the angle beyond the critical angle of water (49°), the red ray of light will not refract but reflect back into the water medium. This is the total

248 LigHt Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 249internal reflection of light through the water. We can use this phenomenon to pass a laser light through the cylindrical column of water. It is used as decoration light in ponds and streams of water around statues and gardens. Medium 1 (Air)Medium 2 (Water)Refracted rayTotal internal reflectionCritical angle Incident rayn n na1. 2. 3.b c d eFig: Total internal reflection in waterIn the schematic diagram given below, a laser light is used to produce an incident ray inside the water. The light is incident at an angle greater than 49°. As a result, the ray of light has suffered total internal reflection many times in the stream and has followed the path of the stream. Fig: Multiple total internal reflections in a stream of waterFibre Optics and Light PipeFibre optics and light pipes are the devices used to transmit light in any desired path. They work because of the total internal reflection. When a ray of light enters a fibre, it suffers a series of total internal reflections and finally emerges out from the other end. During this process, the light energy is not absorbed and it gets transmitted over a long distance. Fig: structure of the optical fibre

250 LigHt Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 251Optical fibreA very thin, transparent tube made from glass or plastic is called optical fibre. Optical fibres are based on the total internal reflection of light. The optical fibre has three parts, the core with a higher refractive index, the cladding with a low refractive index and a lightproof jacket. Core: The innermost flexible transparent thin cylinder of the glass with a higher refractive index is called the core. It works as a denser medium in optical fibre. Cladding: The thin flexible coating around the core with relatively less refractive index than the core is called cladding. It works as a rarer medium. Jacket:The jacket is the outermost protective layer of optical fibre. Working mechanism of optical fibre:The core of the optical fibre has a more refractive index than the cladding so the core acts as denser and the cladding acts as a rarer medium. The light sent at a certain angle suffers multiple total internal reflections. Then it travels to opposite ends of the fibre and transfers data. Optical fibre is used in the medical field. Bundles of many optical fibres are used to make a laparoscope and endoscope. These devices are used for observing internal organs and for keyhole surgery. It is also used for telecommunication. Optical fibre is used for high-speed internet. i) Use of fibre optics in communication: Optical fibre is used extensively as infrastructure for long distance telephone and internet. For this purpose, communication signals like sound are converted into electrical energy and finally into light energy. The light energy confines itself inside the material by total internal reflection and the signal is transmitted over a long distance. By using fibre optics, we can see HD videos, to send and receive large data through internet. FACTS WITH REASONSThe core of the optical fibre has a more refractive index than the cladding. The core of the optical fibre has a more refractive index than the cladding so the core acts as denser and the cladding acts as a rarer medium. The light sent at a certain angle suffers multiple total internal reflections. Then it travels to opposite ends of the fibre and transfers data. ACTIVITY 7Make a 3D model of optical fibre. Optical fibre can transfer data at a minimum speed of 1 Gbps. It can transfer at faster rate than this. MEMORY TIPSFig: optical fibres

250 LigHt Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 251 ii) Use of light pipe for endoscopy and colonoscopy: A light pipe is a flexible fibre that confines a beam of light by total internal reflection. It is used to examine internal organ of human body like stomach, intestine, etc. by physicians. If the light pipe is passed through mouth to observe stomach, food pipe, etc., it is called endoscopy and if light pipe is passed through anus to observe small intestine, large intestine, etc. it is called colonoscopy.Light pipeEndoscope is a device used by physicians to examine the internal organs of the human body passing through mouth.MEMORY TIPSKeyhole surgery The surgical procedure in which the surgeon can access the abdomen and pelvis with the help of a laparoscope is called Keyhole surgery. It is used to remove stones from the kidney and stones from the gallbladder to take out tissue for biopsy, vasectomy, laparoscopy etc. A laparoscope is used for keyhole surgery. A bundle of optical fibre and a camera collectively called a laparoscope. A small cut is made in the abdomen to operate inside the body. A laparoscope is passed through a minor incision. Some bundle of optical fibres send light to organs. The camera takes photos and changes into a light signal. Another bundle of optical fibre brings back light. The injury is seen on the monitor. Other surgical tools also passed through the incision to do surgery. Doctors use that image in the monitor and the surgical tools to perform surgery. Figure of keyhole surgery using laparoscope Fig: endoscopy and colonoscopyKeyhole surgery is also called laparoscopic surgery. MEMORY TIPS

252 LigHt Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 253FACTS WITH REASONSA Laparoscope is used during keyhole surgery. A laparoscope is used during keyhole surgery because it helps to take pictures of internal organs and helps doctors to visualize those organs. ACTIVITY 8Why do doctors prefer laparoscopic surgery? Discuss. MirageA mirage is generated by layers of air at different temperatures. This effect can commonly be seen on asphalt roads and deserts during the summer.It is an optical illusion in which there is an appearance of water mainly on roads, deserts, etc. during the peak summer.Formation of a MirageDuring peak summer days, the black-topped road and the sand on desert absorb solar radiations and make the air above them hot very quickly. This makes the lower layers of air above the road hotter than the upper layers. Cold air is denser than hot air. At the boundary of the denser upper layer and rarer lower layer, light from the blue sky and surroundings bends more and more away from the normal. After several refractions, the angle of incidence becomes greater than the critical angle, which is the condition for the total internal reflection. Hence, light is reflected. The viewer no longer sees the road or desert floor. But the light reflected from the blue sky and surroundings, gives him to an illusion of water. FACTS WITH REASONSWhy does mirage occur?Mirage occurs due to total internal reflection of light on the asphalt roads and deserts during the summer.Sparkling of DiamondDiamond has a special cut of the faces along with its high refractive index of 2.42. Its critical angle is only of 240, which allows total internal reflection over a wide range of angles. Therefore, a lot of incident light undergoes several total internal reflections inside the diamond before exiting through it and causes the “sparkling” effect. This effect makes diamonds so appealing. A well-cut diamond sparkles more than a poorly cut diamond.Virtual imageRarer layer of airDenser layer of airTreeMirageSparkling of diamond

252 LigHt Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 253FACTS WITH REASONSAn air bubble inside water shines. Why?Water is an optically denser medium and air is a rarer medium. When light rays passing through water fall on air bubble inside water at an angle greater than the critical angle then they suffer total internal reflection. These reflected rays enter an observer's eyes. Due to this, the air bubble inside water shines.Sound is not heard at a distance during the day.During the day the air near the ground becomes rarer medium and the air above it becomes denser. The sound travels from rarer medium to denser as we speak. So, it bends away from the normal, bends upward and cannot reach the ears of a nearby person. Hence sound cannot be heard clearly at day.Sound can be heard at a distance during the night.During the night, the air near the ground becomes denser medium and the air above it becomes rarer. The sound travels from denser medium to rare as we speak. So, it bends toward the normal, bends downward and reaches the ears of a nearby person easily. Hence sound is clear at night. Differences between Reflection and Total Internal ReflectionS.N. Reflection S.N. Total Internal Reflection1. The process when an incident ray bounces back to the same medium after falling on a surface is called reflection of light. 1. The process in which an incident ray in a denser medium makes an angle of incidence greater than the critical angle and reflects in the same medium is called total internal reflection of light.2. Angle of incidence can be in any medium. 2. Angle of incidence should be in denser medium. 3. The incident angle can be of any measurement. 3. The incident angle must be greater than the critical angle. DispersionA prism is a piece of transparent material like glass, which is bounded by two triangular surfaces and three rectangular surfaces. The rectangular surfaces are called the refracting faces and the angle between two refracting faces is called angle of prism.The white light consists of seven colors in which the violet color has the least wavelength and red color has the maximum wavelength. Dispersion causes the spreading of all the colors. When white light falls at the first face of an equilateral prism, the red color bends least and the violet color bends maximum. Other colors also bend in their respective order. Thus, when white light falls on the refracting face of a prism it splits up into seven colors and gets dispersed. While passing out from the opposite face of the prism, these seven colors suffer further refraction and separate out further. Thus, the phenomenon due to which white light splits into seven colors (VIBGYOR), when passed through a prism, is called dispersion.White light beamGlass prismDispersion of light through glass prismVIBGYOR

254 LigHt Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 255ACTIVITY 91. Take a prism and keep it on the table. Ensure that the room is considerably dark for the light to be obvious.2. When light travels through the prism, it splits the white light into 7 colors. Observe the colors and check whether you find them in the order of Violet, Indigo, Blue, Green, Yellow, Orange, and Red (VIBGYOR) or not.Cause of Dispersion of White Light by a PrismWhen light touches the surface of the prism, refraction takes place at the boundary of the prism i.e. boundary between air and glass. Then the white light is separated into its component colors, viz. red, orange, yellow, green, blue, indigo, and violet.The seven colors in white light constitute seven different wavelengths. In a glass prism, the speed of the shortest wavelength (the violet color) is slower than the longest wavelength (the red color). The shortest wavelength (the violet color) color bends more than the longest wavelength (the red color). Due to this, the violet color comes at the bottom and the red color comes at top on the screen. Therefore, the difference in speed of seven different wavelength colors inside a prism is the cause of the dispersion of light.Rainbow A rainbow is a semi-circular or circular colourful meteorological phenomenon seen in the sky. It contains seven colours. It is a spectrum because violet, indigo, blue, green, yellow, orange and red colours are arranged based on their wavelength. Conditions required for the formation of a rainbowPosition of the sun: For us to see the rainbow the sun should be behind us. Sun should be at an angle of about 42° from the horizon. Position of water droplet: Tiny water droplets should be scattered in the air between the person and the sun. Dispersion of light: The sunlight enters the water droplet. A water droplet works like a prism. The velocity of light decreases and the light bends (refracts). The colours separate (disperse) because each colour has a different velocity inside the water droplet. Then the light reaches another face of the water drop and reflects within the water droplet. Then the light comes out of the water drop (refraction) again. As a result, a circular rainbow is formed. It appears semi-circular and of the same size when observed from the ground because only the top half of the rainbow is seen from the ground. Recombination of White Light: Newton's Experimentsa) Two-prism experimentIn this experiment, Newton took two prisms P1 and P2 of the same material and having the same refracting angle. He allowed a fine beam of white light to fall on the prism P1. He obtained a spectrum VIBGYOR on the screen. He removed the screen and placed another prism P2 in an inverted position as shown in the given figure. It was observed

254 LigHt Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 255that the light coming out from the prism P2 is white again. The prism P1 that disperses a white light into its constituent colors is called a dispersing prism. The second prism P2 that recombines the seven constituent colors to form a white light is called a recombination prism.White light beamGlass prismVIBGYORWhite spotColoured Beam of light white lightA D EC B FTwo-prism experimentb) Newton's color discIt is a circular metallic disc divided into seven sectors. These sectors are painted with the colors of the solar spectrum. The disc is made capable of rotating about an axis passing through its center. When the disc is rotated at high speed with the help of the handle, the colored sectors are no longer visible. The whole disc appears dull white in color. It proves that white color of sunlight has seven colors. HOT SKILL HIGHER ORDER THINKING SKILL1. Light bends while changing the optical medium.The light bends while changing the optical medium because of a change in the velocity of light. 2. Stars do not twinkle when observed from international space stations. Stars do not twinkle when observed from the international space station because there is no atmosphere around it. The light from the stars does not refract while reaching the international space station. 3. An air bubble inside the water shines when seen from a certain angle. An air bubble inside the water shines when seen from a certain angle because of the total internal reflection of light. The light goes from a denser water medium and tries to enter into the air bubble rarer medium. Most light incident at an angle greater than the critical angle and suffers total internal reflection. Hence, the air bubble inside the water shines. 4. Differentiate between rarer medium and denser medium. The differences between rarer medium and denser medium are:SN Rarer medium SN Denser medium 1 The optical medium in which light travels at a greater velocity than another medium is called a rarer medium.1 The optical medium in which light travels slower than in another medium is called a denser medium.2 The refractive index of a rarer medium is smaller than that of a denser medium.2 The refractive index of a denser medium is greater than that of a rarer medium.3 Light bends away from the normal when entering rarer medium from denser. 3 Light bends toward the normal when entering denser medium from rarer. Newton's color disc

256 LigHt Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 2575. Keyhole surgery is also called laparoscopic surgery. Keyhole surgery is also called laparoscopic surgery because of the use of the laparoscope to visualize internal organs. 6. Objects, as observed from water into the air, appear farther than they actually are. Give the reason. When objects are observed from the water into the air, they appear farther than they are because the refracted beam of light is bent far away from the normal due to refraction of light. 7. Light does not disperse in a vacuum.Light disperses only when different colours have different speeds and they bend at different angles. However, the velocity of every colour of light is equal in the vacuum so the colours of light cannot bend at different angles and cannot disperse in the vacuum.4 STEPS EXERCISE EXERCISESTEP11. Multiple Choice Questions a. What is the cause of the refraction of light?i. change in medium ii. change in velocity iii. change in frequency iv. change in wavelength b. Why refractive index have no unit?i. it is a pure number ii. it is a derived quantity iii. it is a ratio of two similar quantities iv. it is not a physical quantity c. Why does an air bubble under the water shine?i. it is made of silicon. ii. it reflects light like a mirror.iii. total internal reflection of the light iv. dispersal of the light d. Which of the following represents total internal reflection?Light comes back in the same medium at any value of the angle of incidence Light comes back in the same medium only if the angle of incidence is greater than the critical angle Incident ray can be in any medium A BIncident rays can be in denser medium C Di. A ii. Biii. C iv. De. If the colours of light are arranged based on their wavelengthin decreasing order, which one of the following options is correct?i. VIBGYOR ii. VIBYGORiii. BGYVIOR iv. ROYGBIV

256 LigHt Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 257f. A ray of light is passed at an equal angle of incidence from three denser mediums A, B and C. Angle of refraction corresponding to an angle of incidence through A is acute, through B is obtuse and through C is a right angle. These three denser mediums are arranged based on their refractive index. Select the correct one. i. C > B < A ii. B > A < C iii. A > B > C iv. C < B < A g. In the given figure, why is the stream of light glowing?i. refraction of light ii. dispersion of light iii. reflection of light iv. total internal reflection of lighth. If the angle of refraction in the air medium is 90°, when light is passing from glass to the air, calculate the value of the angle of incidence in the glass medium. i. 0° ii. 90°iii. 42° iv. 49°i. Whenever the ray of light refracts through the glass slab when the incident ray is in an air medium, which of the following statements is true?Statement 1: Incident ray and emergent ray are in the same medium. Statement 2: Incident ray and emergent ray have the same velocityi. Statement 1 is true but statement 2 is false.ii. Statement 2 is true but statement 1 is false. iii. Statement 1 and statement 2 are correct. iv. Statement 1 and statement 2 are incorrect. j. Which of the following is keyhole surgery? Surgery Principle Invasive Observed organ i Not required Total internal reflection Yes Stomach ii Not required Total internal reflection Yes Large intestine iii required Total internal reflection Yes Internal organs such as kidney iv Not required Refraction Yes Cornea a. ii b. iii c. iii d. iv e. iv f. ii g. iv h. iii i. iii j. iii Ans of MCQSTEP22. Very short questions a. Kerosene has less density than water but it is called an optically denser medium compared to water. Give reason.b. What happens to the refractive index of glass, if we continuously increase the angle of incidence?

258 LigHt Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 259c. Which one is a denser medium?bends toward normalbends away medium A Medium B from normal refracted ray of lightd. When does light bend away from normal?e. Which one is the rarer medium and why?f. Find the critical angle of the medium whose refractive index is 1.65.g. If the light is passing from a denser medium to a rarer medium and if the angle of refraction is 90°, What will be the angle of incidence equal to? h. On a hot summer day, we can see water puddles in the pitched road but it's not there when we reach there. Explain the reason i. If we have three media: kerosene (µ = 1.44 ), water (µ = 1.33) and turpentine (µ = 1.47). In which of these, the speed of light is maximum?STEP33. Short questions a. Study the given hypothetical story and answer the following questions. [After dinner Ram felt acute pain in his chest and almost fainted. He thought it was a heart attack. He made a wise decision and went to the hospital. Doctors did blood tests and an ECG on him. Thankfully it was not a heart attack. So, they used an endoscope to navigate his alimentary canal. They found that it was an ulcer in the stomach or possibly the first-stage cancer.] What is this technology called? Which principle is the machine based on? How is it carried out? b. Differentiate between keyhole surgery and endoscopy.c. Draw the diagram to show the refraction of light through a glass slab. d. Complete the given diagrams.e. When the incident ray enters the glass slab from the air, it bends slightly toward the normal. What is this phenomenon called? Why does it bend toward the normal?f. If an incident ray is perpendicular to the first face of the prism in which the base is a right-angled isosceles triangle, what will be the angle of incidence? Will the light refract or reflect? Why?Medium BMedium A

258 LigHt Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 259g. Draw the ray diagram to show that when the angle of incidence in the air is 25° and the angle of refraction in water is 18.5°.h. How can fish A see fish B without moving vertical distance?STEP44. Long questions a. When the fibre net is cut open, we cannot see copper wire but a thin flexible, hollow and transparent tube made of either plastic or silica is seen. What is it called? What is its function? On which principle does it work? How does it work? Why is it preferred over copper wire? b. Kunsang was surprised to know that her water bottle, filled with water, kept in the window was making a colour spectrum in the floor. What is that spectrum called? How many colours are there in that spectrum? How is that spectrum formed? Will a spectrum be formed if the bottle is kept in another place anywhere in the sunlight? Why? Why not?c. Dispersion of light is shown in the given figure. (i) Write the name of colours BA and BG. (ii) Why does the ray BG deviate most? (iii) Which colours are found between BA and BG? d. There is a coin in the test tube. Then the test tube is dipped into water in a beaker as shown in the diagram. (i) Can the observer see the coin from the given position? Write with reason. (ii) What happens if water is filled in the test tube? Why?5. Numerical i. The speed of light in a medium 'M' is 0.75 × 108 m/s and that in vacuum is 3 × 108 m/s. Find the refractive index of 'M'. [Ans: 4]ii. Light enters from the air into a diamond, which has a refractive index of 2.42. Calculate the speed of light in a diamond. [Ans: 1.24 × 108 m/s]iii. The speed of light in a vacuum is 3 × 108 m/s. If the refractive index of water is 1.33, what is the speed of light in water? [Ans: 2.25 × 108 m/s]iv. The angle of incidence in a glass slab is 15° and the angle of refraction is 10°. Calculate the refractive index of the glass.♦♦♦

260 Lens Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 261Key terms and terminologies of the unit1. Lens: A lens is a piece of transparent optical material bounded by two refracting surfaces.2. Convex lens: A lens which is thick in the middle and thin at the edges is called convex lens.3. Concave lens: A lens which is thin in the middle and thick at the edges is called concave lens.4. Center of curvature:The center of the spherical surface from which the lens has been cut is called center of curvature.5. Aperture: The maximum portion of the spherical surface from which refraction takes place is called aperture of the lens.6. Radius of curvature:The radius of the spherical surface from which the lens has been cut is called radius of curvature. 7. Optical center: The geometrical center of a lens is called optical center.8. Principal axis: The line joining both the centers of curvature of a lens is called the principal axis. 9. Principal focus: The point on the principle axis at which parallel rays of light meet after refraction (convex lens) or appear to diverge (concave lens) is called principal focus.Sequence of Curriculum Issued by CDC Lens: Introduction and types of lens (concave and convex) Technical terms related to lens (concave, convex, centre of curvature, radius of curvature, principal axis, optical centre, focus, focal length, image, real image and virtual image) Rules of refraction of light through lens Ray diagram of images formed by concave and convex lenses and their nature Power of the lens: introduction, relation of power of a lens with the curvature of the lens Structure of a human eye and process of formation of image in a human eye  Effects of corneal injuries on vision and corneal transplantation Introduction to colour blindness, night blindness and cataract Introduction to defect of vision (myopia and hypermetropia) Causes of defect of vision and measures to correct them (lenses, contact lenses and laser surgery) UNITLens10.2Estimated teaching periods Theory Practical6 2Dr. Sanduk Ruit, born on September 4, 1954, in Olangchung Gola, Taplejung, is a renowned Nepalese ophthalmologist who has transformed cataract surgery, especially in underserved regions, through his innovative methods and unwavering commitment. His groundbreaking contributions, such as the development of costeffective intraocular lenses and the establishment of the Tilganga Institute of Ophthalmology, have restored sight to millions worldwide. Dr. Sanduk Ruit, About the Nepalese Scientist

260 Lens Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 26110. Focal length: The distance between the optical center \"O\" and the principal focus \"F\" is called the focal length of the lens.11. Focusing: The process of getting a sharp image on the screen by shifting the position of the lens is called focusing.12. Image: An image is a physical likeness or representation of an object produced by a lens after refraction.13. Real image: The inverted image which is formed by the actual meeting of the refracted rays and can be obtained on the screen is called real image.14. Virtual image: The erect image which cannot be obtained on the screen is called virtual image.15. Power of lens: The converging or diverging capacity of a lens is called power of lens. Mathematically, the reciprocal of the focal length in meter is called power of a lens.16. One dioptre: The power of a lens whose focal length is one meter is called one dioptre.17. Magnification: The ratio of the height of the image formed by a lens to the height of the object is called magnification of the lens. Or, the ratio of the image distance (v) to the object distance (u) is called magnification.18. Optical instruments: The devices that use lenses or mirrors to work with light to make images are called optical instruments. 19. Far point: The farthest point up to which an eye can see objects clearly without any strain is called the far point.20. Near point: The nearest position at which the eye can see tiny objects clearly without any strain is called the near point of the eye.21. Least distance of distinct vision:The distance of the near point from the eye is called the least distance of distinct vision. 22. Range of vision: The distance between the near point and the far point is called the range of vision.23. Accommodation: The process in which the thickness of eye lenses is changed by ciliary muscle so that the eyes can focus at the point at the range of vision is called accommodation of the eye. 24. Myopia: The defect of vision in which eyes can focus on nearer objects but cannot focus on distant objects is called myopia or short-sightedness. 25. Hypermetropia: The defect of vision in which eyes can focus on distant objects but cannot focus on nearer objects is called hypermetropia or long-sightedness. 26. Corneal injury: The physical damage to the cornea due to scratches, chemicals or diseases that affect the vision of the eye is called corneal injury.27. Cornea transplantation: The transfer of healthy cornea from donor to the receiver is called cornea transplantation.28. Colour blindness: The inability to see particular colours or their shades is called colour blindness.29. Night blindness: The inability to see properly in poor light conditions is called night blindness.30. Eye cataract: A disease in which the eye lens becomes cloudy due to the deposition of protein crystals on it is called eye cataract.31. Contact lenses: The contact lenses are transparent curved and soft lenses that are put on the eyes directly.

262 Lens Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 263IntroductionWe see a variety of objects in our surroundings. This is because light travels from the luminous source to the object and then comes from them to our eyes. During the day, the sunlight falls on the objects and reflects from them. The reflected light reaches to our eyes and helps to see the objects. Thus, light is a form of energy which produces a sensation of vision. Light travels in the same direction in all media. If light enters obliquely from one transparent medium to another, the direction of propagation of light in the second medium changes. This phenomenon of bending of light as it passes obliquely from one optical medium to another is called refraction of light. Eyes, camera, microscope, telescope are some familiar optical instruments with lenses. In this unit, we will study about the properties of lens and their application in optical instruments. We also describe the structure, types of defect of human eyes and the ways to correct them. LensA lens is a piece of transparent optical material bounded by two refracting surfaces. The refracting surfaces of the lens are usually spherical. Lenses are generally made from plastics or glass. Refraction Through LensesIf light enters obliquely from one transparent medium to another, the direction of propagation of light in the second medium changes. Lens can change the size of an image with the help of refraction of light. Lenses are used in making many optical instruments like camera, compound microscope, telescope, etc. on the basis of the refraction of light through a lens. A person with defect of vision wears spectacles with lens of certain power. Types of LensesLenses are made of a great variety of shapes from different kinds of glasses and plastics. But, they belong to either converging group or diverging group. Therefore, on the basis of uses, there are two types of lenses. They are:i. Convex lens or converging lens ii. Concave lens or diverging lensi. Convex LensA lens which is thick in the middle and thin at the edges is called convex lens. It is bulged out at the center. It converges the parallel rays of light at a point after refraction.Convex lens is called a converging lens, why?A convex lens converges the parallel rays of light at a point after refraction. So, it is called a converging lens.FACTS WITH REASONS

262 Lens Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 263Types of convex lens: There are three types of convex lenses. They are:1. Biconvex or double convex lens: Its both surfaces are convex.2. Planoconvex lens: It has one surface convex and the other surface plane.3. Concavo convex lens: It has one surface convex and the other surface concave. Double convex Plano convex Concavo convexMEMORY TIPSACTIVITY 11. Take a convex lens and a piece of paper. 2. Focus the sun light to refract directly through the magnifying glass in a straight line onto the paper piece. 3. Tilt the magnifying glass until the focused light becomes a perfect circle.4. Move the magnifying glass towards or away from the paper, until you find a pin point spot. The light rays get converged at that spot. 5. Heat the paper piece until it catches fire. Conclusion: This activity proves that convex lens converges the light rays on a paper piece. On excessive heating it starts to burn. A convex lens is made from the group of prisms: converging action of a convex lensIt is considered that, a convex lens is made up of a large number of prisms. The prisms in the upper half of the lens have their bases downward and the prisms in lower half have their bases upward. The angle of prism goes on decreasing from the central line. The central part of the prism is just like a glass slab.In case of a prism, the lower the angle of a prism, the greater the bending of light. Thus, the light rays falling on the edges of a convex lens bend the maximum. Finally, all the rays meet at a point after refraction. This point is called the principal focus point of the convex lens.A convex lens converges light rays.A convex lens can converge light because a convex lens is a combination of many prisms in such a way that the base face of the prism is toward the optical centre. The refracted rays are bent toward a single point. FACTS WITH REASONSii. Concave LensA lens which is thinner in the middle and thicker at the edges is called concave lens.It is depressed at the center and bulging out at the edges. A concave lens diverges the parallel rays of light. Converging lens

264 Lens Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 265A concave lens diverges the rays of light.A concave lens can diverge light because a concave lens is the combination of many prisms in such a way that the edge of the prism is toward the optical centre. So refracted ray moves away from each other. FACTS WITH REASONSTypes of concave lens: There are three types of concave lenses. They are:1. Biconcave or double concave lens: Its both surfaces are concave.2. Planoconcave lens: It has one surface concave and the other surface is plane.3. Convexo concave lens: It has one surface convex and the other surface is concave. Double concave Plano concave Convexo concaveMEMORY TIPSA concave lens is made from the group of prisms: diverging action of a concave lensIt is considered that a concave lens is made up of large number of prisms. The prisms in the upper half of the lens have their bases upward and the prisms in lower half have their bases downward. The angle of prism goes on increasing from the central line towards the edges of the lens. The central part of the prism is just like a glass slab.The light rays passing through a prism deviate towards its base. The smaller the refracting angle, the greater the deviation produced. Thus, the light rays near the edges of a concave lens deviate less but the light rays around the central line of the lens deviate more. When all the diverged light rays are produced backward, then they meet a point. This point is called the principal focus of a concave lens.A concave lens has a virtual focus, why?The refracted rays from a concave lens appear to diverge from a point. Refracted ray does not meet physically but appears to do so if it is produced backward at a point. So, a concave lens has a virtual focus.FACTS WITH REASONSTerminology Related To Lensa. Center of curvatureThe center of the spherical surface from which the lens has been cut is called center of curvature. Two spherical surfaces of a lens are the parts of two spheres. So, there are two centers of curvature. In the given figure, C1 and C2 are the centers of curvature of two spherical surfaces that form a lens. b. ApertureThe maximum portion of the spherical surfaces from which refraction takes place is diverging lens

264 Lens Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 265called aperture of the lens.Center ofcurvature (C1)Center ofcurvature (C1)2F F FO2FX Y X YPrincipalaxis2F F F 2FCenter ofcurvature (C2)Center ofcurvature (C2)OA AB BConcave lens Convex lensR R R RAperture of convex lens Aperture of conave lensc. Radius of curvatureThe radius of the spherical surface from which the lens has been cut is called radius of curvature. In the given figures, OC1 and OC2 are the radii of the curvature of the two lens surfaces. It is denoted by the letter 'R'.d. Optical centerThe geometrical center of a lens is called optical center. An optical center is at an equal distance from either sides of a lens. It is inside the lens. It is denoted by the letter 'O'.e. Principal axisThe line joining both the centers of curvature of a lens is called the principal axis. In the given figure, C1C2 is the principal axis.f. Principal focusIn case of a convex lens, the point on the principle axis at which parallel rays of light meet after refraction is called principal focus. It is a real focus. But for a concave lens, the point on the principle axis from which parallel rays of light appear to diverge after refraction is called principal focus. It is a virtual focus. There are two foci, one on each side of the lens. They are at equal distance from the optical center.(Focus)FParallel raysof lightPrincipal axisF'(Focus) (Focus)(Focal length)(a) (b)Ff(Focal length)FocusPrincipal axisParallel rays of lightConcave lensor diverging lens Convex lensor converging lensfO F OPrincipal focus of convex lens and concave lensg. Focal lengthThe distance between the optical center \"O\" and the principal focus \"F\" is called the focal length of the lens. It is denoted by the letter 'f'. The distance of real focus in convex lens is taken as positive and that of virtual focus in concave lens is taken as negative.Focal length of a lens is half of its radius of curvature, i.e. f = R2.MEMORY TIPS

266 Lens Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 267ACTIVITY 2To determine the approximate focal length of a convex lens1. Take a convex lens and a cardboard.2. Put a convex lens in a holder or a stand.3. Keep the lens infront of a distant object, such as tree, so that the rays coming from the tree pass through the lens.4. Put a cardboard as a screen behind the lens. 5. Adjust the distance of the screen from the lens until a clear inverted image of the tree is formed on the screen.6. Measure the distance between screen and the lens. This distance will be the approximate focal length of the convex lens.Focusing: Photographers adjust the position of the lens before clicking a photo through their camera. This is done to obtain a sharp image. Thus, the process of getting a sharp image on a screen by shifting the position of the lens is called focusing.Rules of drawing ray diagramsRule 1If a ray of light parallel to the principle axis incidents on a convex lens, it should pass through a focus on another side of the lens after the refraction. If a ray of light parallel to the principle axis incidents on a concave lens, it does not pass through focus but looks like it is diverging from the focus of the same side. F O F OConvex lens Concave lensRule 2A ray of light passing through the focus of a lens becomes parallel to its principal axis after refraction through the lens. F O F OConvex lens Concave lensRule 3A ray of light passing through the optical center of a lens does not deviate after refraction through the lens.

266 Lens Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 267O OConvex lens Concave lensImage and Its TypesAn image is a physical likeness or representation of an object produced by a lens after refraction. The rays of light from the object fall on the lens and get refracted. These refracted rays produce a picture of the object called image. Lenses form two types of images. They are:i) Real image : Real image is formed when the refracted rays actually meet at a point. This image can be obtained on the screen. It is always inverted. A convex lens generally forms a real image. Thus, the inverted image which is formed by the actual meet of the refracted rays and can be obtained on a screen is called a real image.ii) Virtual image : Virtual image is formed when the refracted rays appear to meet at a point. This image cannot be obtained on a screen. It is always erect. A concave lens generally forms a virtual image. Thus, the erect image which cannot be obtained on a screen is called virtual image.Images Formed by a Convex LensA convex lens can form both real and virtual images. The type of image formed by a convex lens depends on the position of the object in front of the lens. The object can be placed at different positions to get different types of images. The various types of images formed by convex lens are described below.ACTIVITY 31. Take a convex lens and place it on the stand. 2. Now, light a candle and place it at some distance in front of the lens.3. Place a paper screen on the other side of the lens. 4. Change the position of the object from the lens as you observe the change in shape and size of the image.Convex lens CandleScreen with candleFor simplicity, rays are usually shown bending along the upright line through the middle of the lens, though in reality bending takes place at each point of the surface.MEMORY TIPSThe concave lens forms virtual images only but the convex lens can form real images as well as virtual images based on the position of the object from the lens. MEMORY TIPSA convex lens can form both real and virtual images depending upon the distance between object and lens.MEMORY TIPS

268 Lens Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 2691. When object is at infinityCharacteristics of the image i. Realii. Invertediii. Diminished to a pointiv. Formed at focus 'F' on the other side of the lensApplication : This type of image formation is used in burning glasses, astronomical telescope and camera2. When object is beyond 2FCharacteristics of the imagei. Realii. Invertediii. Diminishediv. Formed between F and 2F on the other side of the lensApplication : The camera is based on this type of image formation.3. When object is at 2FCharacteristics of the imagei. Realii. Invertediii. Equal to the size of the objectiv. Formed at 2F on the other side of the lensApplication : Erecting lens of the telescope makes this type of image. 4. When the object is between F and 2FCharacteristics of the imagei. Realii. Invertediii. Magnifiediv. Formed beyond 2F on the other side of lensApplication : Projectors use this type of image.5. When the object is at the focus FCharacteristics of the imagei. Realii. InvertedFB'DA' Image(Real, inverted)O F'BA Parallel rays from top point of a distant object2fImage(Real, inverted)Convex lensB 2F' F'AO FA'B'2F2fF OF2FAB B'A'2FImage(Real, inverted)ObjectF' C F 2FA'B'2F ObjectImage (Real, inverted and magnified)A DBX Image at infinityRays become parallelObjectConvex lensFAOFYBf

268 Lens Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 269iii. Highly magnifiediv. Formed at infinity, on the other side of the lens.Applications : Flashlights and laser lights use this type of image. If a bulb is kept in focus of a convex lens then it produces a parallel beam of light.6. When the object is between the optical center (O) and focus (F )Characteristics of the imagei. Virtualii. Erectiii. Enlarged (or magnified)iv. The image is formed on the same side of the object (beyond the object).Applications : A simple microscope and hand lens make this kind of image. It is used to see tiny objects such as parts of insects, floral parts, read books etc.Summary of the Images Formed by a Convex LensLocation of the object Location of the image Size of the image Nature of the image1. At infinity At focus (F) Highly diminished Real and inverted2. Beyond 2F Between F and 2F Diminished Real and inverted3. At 2 F At 2 F Same size as the object Real and inverted4. Between F and 2F Beyond 2F Magnified Real and inverted5. At F At infinity Highly magnified Real and inverted6. Between F and O Image formed on the same side of the object Magnified Virtual and erectImage Formed by a Concave LensConcave lens forms diminished, virtual and erect image when an object is placed anywhere between the optical center and infinity. The image lies in between the principal focus and optical center on the same side of the lens. 1. When the object is at infinityCharacteristics of the imagei. Virtual.ii. Erect.iii. Highly diminished to a point sizeiv. Formed at focus on the same side of the objectApplication : This type of image formation is used as an eye lens in Galileo Telescope. and to treat short sightedness.Image (virtual, erect and enlarged)Convex lensO FBA' F AB'ImageObjectThe image formed by a concave lens is upright, virtual and diminished for any object regardless of the distance between the object and lens.MEMORY TIPSF O F

270 Lens Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 2712. When the object is anywhere between the optical center and infinityCharacteristics of the imagei. Virtualii. Erectiii. Diminishediv. Formed between the optical center and principal focus on the same side of the objectApplications : This type of image formation is used for the peepholes at the doors.Differences between convex lens and concave lensS.N. Convex lens S.N. Concave lens1. The lens thick at the mid and thin at the edges is called a convex lens. 1. A lens thin at mid and thick at the edges is called a concave lens.2. It is a converging lens. 2. It is a diverging lens. 3. It has real focus and real focal length. 3. It has a virtual focus and a virtual focal length.4. It has positive power. 4. It has negative power.5. It is used to treat long-sightedness. 5. It is used to treat short-sightedness.Power of a lensThe converging or diverging capacity of a lens is called power of a lens. If a convex lens converges a parallel beam of light closer to its optical center, then the focal length is less but the converging power is high. Similarly, if a concave lens diverges a parallel beam of light from closer of its optical center, then the focal length is less but the diverging power is high. Thus, the power of a lens depends upon its focal length.The reciprocal of the focal length in meter is called power of a lens. Mathematically,Power of lens (P) = 1focal length (in metre) = 1fA convex lens has a positive focal length. So, the power of a convex lens is considered to be positive, whereas the focal length of a concave lens is negative. So, the power of a concave lens is considered to be negative.Concave lensXEyeCenter line of the lens(Virtual, erect and diminished) ObjectImageF B'A'B C YAConcave lenses are very similar to convex mirror in their image forming properties.MEMORY TIPSA lens of short focal length has more power, whereas a lens of long focal length has less power.MEMORY TIPS

270 Lens Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 271A thick lens has high power. A thick lens has high power because a thick lens has a short focal length. Power is reciprocal of focal length. So, if the lens is thick, the focal length will be short and hence the power of the lens will be greater. FACTS WITH REASONSUnit of the power of a lensIf the focal length of a lens is measured in meter, then the unit of power is meter-1. It is also called dioptre and is denoted by the symbol \"D\".One dioptre powerThe power of a lens whose focal length is one meter is called one dioptre.Solved Numerical 10.2.1A concave lens has a focal length of 50 cm. Calculate its power.Solution: Given,The focal length of a concave lens (f) = - 50 cm [since the focal length of concave lens is negative]or, f = –50100 m = – 0.5 mNow, the power of a lens, P = 1– 0.5 = –2 DSolved Numerical 10.2.2A convex lens forms a real and inverted image of an object at a distance of 50 cm. How far is the object from the lens if the image is equal to the size of the object? Also, calculate the power of the lens.Solution: Given,Image distance = 50 cmIf the image size is equal to the object size, then the object is at a distance double of the focal length (i.e. 2f)In this case, image distance = object distance = 2f = 50 cmor, f = 25cm = 25100 m = 0.25 m Now, power of a lens, P = 10.25 = 4 DThus, the power of the convex lens is +4 D.Solved Numerical 10.2.5A convex lens has focal length 2 cm. If a burning candle is kept at 3 cm from the optical centre of the lens, draw the ray diagram and write down the nature of the image formed. Calculate the power of the lens. Also, write down one application of this lens.

272 Lens Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 273The ray diagram of the image formed is shown in the given figure.Nature of the image : Real; Inverted; MagnifiedFormed beyond 2F on the other side of lens.Solution: Given, Focal length (f) = 2 cm = 0.02m Power of lens (P) = 1f = 10.02 = 50 DApplication: This type of image formation is used in film and slide projectors.Optical InstrumentsCamera, magnifying glass, telescope, microscope, binoculars, human eyes, etc. use one or more lenses. They are called optical instruments. Most of the optical instruments work on the basis of refraction and reflection of light. Thus, instruments that are used to produce an image of an object with the help of a lens or lenses are called optical instruments.The Human EyeEyes are very important natural optical instruments to see the wonderful world and colours around the animals. Human eyes can focus automatically on the objects over a wide range of distance. The important parts of human eye are: i) Sclerotic : It is also called the 'white of the eye'. It protects the eyeball.ii) Cornea : Cornea is a protective transparent membrane at the front surface of the eye. It has a curved surface which acts as a convex lens. Light refracts when going through the cornea into the fluid between cornea and lens.iii) Choroid : Choroid is the middle layer of the eyeball. It absorbs excess light in the light. iv) Iris : The iris controls the amount of light reaching to the retina. In bright light, iris expands making the pupil narrower. In dim light, iris contracts, widening the pupil.v) Eye lens : Eye lens is a natural convex lens consisting of layers of F' C F 2FA'B'2F ObjectImage (Real, inverted and magnified)A DBThe relation between the object distance (u), image distance (v) and focal length (f) of a lens is called the lens formula. It is given as,1f = 1u + 1vHere, f = focal length, v= image distance and u = object distance. This formula can be applied both in convex and concave lenses.MEMORY TIPSHuman eye is an optical instrument similar to a photographic camera.MEMORY TIPSCornea of the eye is reused when one donated the eye. The cornea of the donor is removed after the death of the person and transplanted to a blind person whose original cornea is opaque. Thus, the blind person gets sight.MEMORY TIPSPupil Iris

272 Lens Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 273transparent fibers. It is covered by a clear membrane. The main function of eye lens is to focus image on the retina. Eye lens is flexible and attached to the ciliary muscles. In order to focus the objects at different distances, the shape of the eye lens and the focal length changes by contracting or relaxing the ciliary muscle of the eye. vi) Ciliary muscle : Ciliary muscle makes the eye lens thicker and thinner. To see close objects, the ciliary muscle makes eye lens thicker and more curved. The eye lens becomes thinner and less curved while looking at distant objects. It works with suspensory ligaments to change the thickness of lens.vii) Pupil : The black coloured spot which is present at the centre of the eye is called pupil. It controls entering of light into the eye. Its size increases and decreases depending upon the brightness of the light. Similarly, the size of pupil is controlled by iris muscle. viii) Aqueous humour and vitreous humour : These are the two types of liquids in our eyes. Aqueous humour is present in between cornea and lens. It makes the bulged shape of the cornea. Vitreous humour is present in between the lens and retina. It retains the shape of the eye ball. Light rays bend as they pass through these liquids which help in focusing. ix) Retina : It is the innermost layer in our eye. A real, inverted and diminished image is formed on the retina. There are light sensitive rod cells and cone cells on the retina to receive the stimulus of light. The image formed on the retina is conveyed to the brain by the optic nerve and gives rise to the sensation of vision. x) Optic nerve : The light-sensitive cells present in the retina get activated and generate electrical signals. The retina sends these electrical signals to the brain through the optic nerve and gives rise to the sensation of vision. The mind interprets the inverted image formed on the retina as an erect object.IrisScleroticRetina Choroid Optic nerveSuspensory ligamentCiliary musclesCorneaAqueous humourStructure of human eyeThin lens for distance visionThick lens for near vision

274 Lens Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 275Range of Vision and Accommodationi) Far PointThe farthest point up to which the eye can see objects clearly without any strain is called the far point. For a normal healthy eye, the far point is at infinity, but for a defective eye, it may be at a finite distance.ii) Near PointThe nearest position at which the eye can see tiny objects clearly without any strain is called the near point of the eye. Generally, the near point for a healthy eye is at 25 cm.iii) Least distance of distinct visionThe distance of the near point from the eye is called the least distance of distinct vision. In other words, the minimum distance at which the eye can see objects distinctly without strain is called least distance of distinct vision. iv) Range of visionThe distance between the near point and the far point is called the range of vision.v) Power of accommodationEye lens becomes thick when we want to see near objects. In this case, focal length of the eye lens reduces to form image of an object on the retina. In the same way, the eye lens becomes thin with increasing focal length to see distant objects. Thus, the ability of an eye to focus the image of objects at various distances on the retina by changing the focal length of the eye lens is called power of accommodation. Ciliary muscles are relaxedImageParallel rays from a distant objectObject at infinityHere eye lens has large focal length but small converging powerlens focusing at far pointHere eye lens has short focal length but large converging powerImageCiliary musclesare tenseDiverging rays from nearby object25 cmOlens focusing at near pointRole of the Ciliary muscle in the power of accommodation Ciliary muscles are very flexible muscle fibres that hold the lens and change its shape during accommodation. Ciliary muscles help the human eye attain the power of accommodation.Our eye can form the image of an object on the retina from near and far points by changing the focal length of the lens. The focal length of the lens is changed by the ciliary muscle. If the object is near, the ciliary muscle contracts. The lens will be curved and thick. Focal length becomes short and focuses on near objects. If the object is far away, the ciliary muscle relaxes. The lens becomes flatter and thinner. The focal length will increase and focus on farther objects.

274 Lens Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 275If the ciliary muscle stops working, then the eye will have difficulty focusing because the thickness of the lens cannot be changed properly. If the ciliary muscles cannot relax, the lens cannot become thinner, so it remains thick with a short focal length. Then a person suffers from short-sightedness. If the ciliary muscles cannot contract, the lens cannot become thicker, so it remains thin with a longer focal length. Then a person suffers from long-sightedness. Every image formed by the eye lens is formed on the retina because the image distance of the eye lens is almost constant. It is possible to make every image at the retina because the thickness and curve of the lens can be changed. The ciliary muscle helps the eye to change the thickness of the eye. This activity is called the accommodation of the eye by focusing.The pupil grows larger in the evening but smaller in the noon if we are outdoors. The pupil grows larger in the evening to collect as much light as possible for clearer vision.Pupil grows smaller in the noon if we are outdoors to reduce the amount of light entering the eye. Our normal eyes cannot focus on objects closer than 25 cm. Our normal eyes cannot focus on objects closer than 25 cm because the focal length of the eye lens cannot decrease further than 25 cm. To see closer than 25 cm the eye lens should curve more and become thicker so that its focal length will decrease but it cannot. FACTS WITH REASONSCorneal InjuryThe cornea is the front transparent part of the eye. Its refractive index is 1.376 and its power is 43 D. It acts as a main lens of the eye. It can converge the beam of light so that an image will be formed in the retina. Sometimes, the cornea can be hurt due to rubbing, scratching, dust, sharp objects, viruses, diseases, germs, fungus etc. It is called corneal injury. The physical damage to the cornea due to scratches, chemicals or diseases that affect the vision of the eye is called corneal injury. It can blur the vision. Corneal injuries can cause permanent blindness. If a corneal injury, such as an ulcer is not treated on time, it could cause permanent blindness. Keratoconus makes the cornea conical and causes short-sightedness in the beginning. Later it can cause blindness too.Corneal injury can cause the following problems:a. Corneal ulcer (keratitis): Corneal ulcer also called keratitis is an open sore on the cornea due to microbial infection. Bacterial and fungal infections cause corneal ulcers. It could lead to red eyes, water eyes, blurred visionb. Corneal abrasion: Injury in the cornea from scratch, pressure, sharp objects etc is called If the cornea becomes flatter than usual, it causes long-sightedness. If the cornea becomes more curved than usual, it causes shortsightedness. MEMORY TIPSFigure to show corneal injury

276 Lens Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 277corneal abrasion. Sand, dust and contact lenses can cause corneal abrasion. It leads to sensitivity to light, infection, tears etc. c. Corneal edema: The defect of vision in which the fluid present between the layers of the cornea and vision becomes blurry is called edema. If there is a scratch in the cornea and if the liquid is stored between the layers, then the vision will be blurred. d. Keratoconus: The defect of vision in which a cornea loses its hemispherical shape to a conical and a person suffers from short sight is called keratoconus. If the cornea becomes conical due to infection or injury it causes short sight or permanent blindness. We should not rub our eyes.We should not rub our eyes because rubbing the eyes continuously can scratch our eyes. FACTS WITH REASONSACTIVITY 4Make a model to represent corneal transplantation.Corneal transplantation Corneal injury usually heals within a day. However, some corneal injury can take longer time. If a corneal injury does not heal on time or never heals, it can cause permanent blindness too. In such cases, we have to replace the damaged cornea with the healthy cornea. The process of treating corneal blindness by replacing a damaged cornea with a healthy cornea is called corneal transplantation. The cornea can be donated after death. It is used for corneal transplantation to treat corneal blindness. The eye donation does not mean giving the entire eye. If the person has demised, they can donate the cornea part of the eye. Eyes should be closed until it is harvested to prevent drying. The doctors will check and then cut a very small portion of the cornea with a minor surgery within 8 to 12 hours from demise. It is a very small, bloodless and tiny surgery. An eye bank is a facility where the harvested cornea is stored. Tilganga Eye Institute of Nepal has an eye bank. It helps in harvesting, storing and distributing cornea for corneal transplantation. Figure to represent cornea transplantation Fig: corneal abrasionWe should not rub our eyes because rubbing the eyes continuously can scratch our eyes. MEMORY TIPSThe cornea is harvested from donors after their demise, within 8 to 12 hours for better quality.MEMORY TIPS

276 Lens Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 277ACTIVITY 5Watch a video on corneal transplantation. Colour blindness For a person to see a particular colour of an object, let’s say red, that object must reflect the red colour of the sunlight and the red cone cells should be receiving that red light. Cone cells help to separate various colours of the object. If those cone cells do not work properly, we cannot see particular colours. Mainly colour blindness is heredity. Mutation and damage in blue cone cells, red cone cells and green cone cells due to harmful radiations etc. can also cause colour blindness. A person with colour blindness cannot see a particular colour or its shades. Therefore, the inability to see particular colours or their shades is called colour blindness. The Ishihara test is done to diagnose colour blindness. Deuteranopia is the inability to see red and green colours. It is also called red-green colour blindness. It is caused by heredity and damage in red cone cells and green cone cells. ACTIVITY 6Observe the given Ishihara test. Try to read the number. If we see the number 21, we may be colour-blind. Number 74 is written in there.Night blindness The inability to see properly in poor light conditions is called night blindness. Night blindness is caused by a lack of vitamin A, diseases, lack of nutrients, short sight, heredity etc. Rhodopsin is a light-sensitive pigment in rod cells. Rhodopsin helps to see in dim light also. Rhodopsin is made of protein and vitamin A. If vitamin A is lacking, rhodopsin will not be formed. If rhodopsin is lacking rod cells cannot work properly. The cells in the retina that help in low-light vision are called rod cells. So, the person cannot see in dim light. Night blindness is also called nyctalopia. Children should eat enough green vegetables to avoid night blindness. Children should eat enough green vegetables to gain sufficient nutrients including vitamin A. Vitamin A will be used to make rhodopsin. It helps to make rod cells. Then children will not suffer from night blindness. FACTS WITH REASONSThe retina contains millions of cone cells which are broadly divided into three types. They are red cone cells, green cone cells and blue cone cells.MEMORY TIPSFigure of source of vitamin A

278 Lens Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 279Eye cataract A disease in which the eye lens becomes cloudy due to the deposition of protein crystals on it is called eye cataract. If a protein-crystal deposits on the lens the lens cannot refract the light properly. So, the vision becomes blurred. It appears like a permanent blindness. If more crystals deposit on the lens, the lens grows opaquer and could cause permanent blindness. The lens on which the protein crystal is deposited appears like a grey dark patch on the pupil. An eye cataract is caused due to old age, smoking, diabetes and ultraviolet radiation. It is easily treated by eye cataract surgery using intraocular lens. Intraocular lens:The artificial lens made of silicon or acrylic is called an intraocular lens. It is used to replace the clouded lens of the eye. It helps to treat blindness due to eye cataracts. Dr. Sanduk Ruit is referred god of sight by NBC News because he developed an intraocular lens in 1995 AD. It could treat blindness due to eye cataracts at a very low cost. This technology has treated the blindness of thousands of people. ACTIVITY 7Collect some pictures of eyes with eye cataracts and make a collage to raise public awareness.Eye cataract surgery Eye cataract surgery is surgery to treat blindness caused by eye cataracts where a clouded lens is replaced by an intraocular lens. It is done by a minor surgery. A tiny cut is made on the side of the cornea. The clouded lens is broken into tiny pieces with the help of the ultrasound. Broken pieces of the clouded lens are now sucked and removed by a vacuum tube. Then the intraocular lens is placed in the eye. Then the cornea is stitched. Hence, intraocular surgery is completed. The benefits of intraocular lenses are cheaper, higher success rate, long-lasting etc. The role of intraocular lenses developed by Nepal’s ophthalmologist Dr. Sanduk Ruit in the treatment of cataractsa. Restoring vision: this lens replaces the clouded lens and restores visionb. Affordable: The intraocular lens made by Dr. Sanduk Ruit is cheap. So, it is affordable for all. So many people are encouraged to go for treatment. So many people have recovered vision throughout the world. c. Simple technology: Here, a small cut is made on the side of the cornea, ultrasound is used to break the clouded lens and the intraocular lens is put. Technology is simple, with no major cuts in the body, painless and new doctors can easily learn it. Eye cataract surgery was very expensive but now it has become affordable because Dr. Sanduk Ruit made an intraocular lens in 1995.MEMORY TIPS

278 Lens Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 279Figure of eye cataract Figure of eye cataract laser surgeryDefects of Vision and Their CorrectionObjects in the range of vision are visible to us due to power of accommodation. The eye may gradually lose its power of accommodation. In such condition, a person cannot see the objects clearly and comfortably. He/she feels strain in the eyes and the vision becomes blurred. The main two types of defects of vision are myopia (or short-sightedness) and hypermetropia (or long-sightedness). a) Myopia or Short-sightednessIn case of a normal eye, the light rays from the objects fall on the eye and converge on the retina. But in case of a myopic eye, the light rays from a distant object falling on the eye are focused at a point in front of the retina in the vitreous humor. So, this person can see near objects clearly but cannot see the distant objects clearly. Thus, myopia is the defect of vision in which a person can see near objects clearly, but cannot see distant objects.For example, a student with myopic eye sitting on the back bench in the classroom cannot read the writing on the board clearly though he can read his book comfortably.Causes of myopia or short sightednessThe two causes of myopia or short-sightedness are:i) The focal length of the eye lens becomes too short : This means while viewing distant objects, the focal length of the eye lens becomes shorter than the diameter of the eye ball. In such conditions, the crystalline lens in our eye is not able to focus the rays on the retina. In case of an eye suffering from myopia, the refracted rays converge in front of retina to make image, and cannot see distant objects?If we have short sight we cannot see distant objects clearly because the parallel rays from distant objects do not meet at the retina. They meet slightly in front of the retina because the ciliary muscle cannot increase the focal length of the long-sighted lens. FACTS WITH REASONSThe loss of power of accommodation of an eye results in the defect of vision.MEMORY TIPSDr. Sanduk invented a very cheap and high quality intraocular lens in 1995. The lenses are being used to treat eye cataract in Asia and Africa.MEMORY TIPSSign of myopia: People rub their eyes when they are reading or looking at the screen. They want to see clearly.In case of a myopic eye, the distant vision is blurry without wearing spectacles having concave lenses of suitable focal length.MEMORY TIPS

280 Lens Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 281ii) Elongation of the eye ball : When the eye ball increases, the distance between retina and eye lens becomes more. In such condition, the image of the object is formed before the retina.Correction of myopia or short-sightednessTo correct myopia, the person has to use spectacles with a concave lens of suitable focal length. The light rays from a distant object are diverged by the concave lens and converged by the eye lens to form a final image on the retina.Concave lensImage ImageEye lensAffected by myopia Correction of myopiab) Hypermetropia or long-sightednessThe defect of vision in which a person can see distant objects clearly but cannot focus on nearby objects is called long-sightedness. For example, a person having the defect of hypermetropia cannot read a book clearly though he can read the distant sign boards. In case of hypermetropia the eye lens focuses the incoming light rays at a point behind the retina. Causes of hypermetropia or long-sightednessThe two causes of hypermetropia are:i) The focal length of the eye lens becomes too long : While viewing the near objects, the focal length of the eye lens becomes more than the diameter of the eye ball. In such condition, the eye lens is not able to converge the rays sufficiently on the retina. The refracted rays converge behind the retina in case of an eye suffering from hypermetropia, and cannot see near objects?If we have long sight we cannot see closer objects because rays of light from nearer objects do not meet at the retina. They meet slightly behind the retina because the ciliary muscle cannot decrease the focal length of a short-sighted lens. FACTS WITH REASONSii) Shortening of the eye ball : Shortening of the eye ball decreases the distance of retina from the eye lens. In this condition, the image of an object is formed behind the retina.Correction of hypermetropia or long-sightednessTo correct a hypermetropia, the person has to use spectacles with a convex lens of suitable focal length. The convex lens can converge the parallel beam of light. If a convex lens of Sign of Hypermetropia: People can't read a menu, book or newspaper without holding it at arm's length.The near point of a hypermetropic eye is more than 25 centimeters away. MEMORY TIPS

280 Lens Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 281Figure of contact lensessuitable focal length is used, it can help to converge the rays of light from infinity at the retina. So, that the person can see clearly. Convex lensImageEye lens Eye lensImageAffected by hypermetropia Correction of hypermetropiaContact lenses The contact lenses are transparent curved and soft lenses that are put on the eyes directly. It is put just above the cornea. Contact lenses are very effective for the treatment of defects in vision because they can correct the defect of vision with less power. They improve peripheral vision and remove distortion of vision because contact lenses move along with the cornea. We should not sleep with contact lenses in the eye because it can cause infection, ulcers and discomfort. We should wash our hands before putting on and removing contact lenses to prevent infection. Comparison between the use of spectacles and the use of contact lenses to correct visual defectsUse of spectacles to correct visual defects Use of contact lenses to correct visual defects• Spectacles are larger. • Spectacles are larger. • They should be dry. • They should not be sterilised. • They are far away from the eye lens. • For the same eye, spectacles must have more power.• Images could be distorted.• Does not improve peripheral vision.• Spectacles are easy to use.• It does not affect the cornea.• Contact lenses are smaller in size.• They should be moist. • They should be sterilised. • They are nearer to the eye lens.• For the same eye, contact lenses have less power.• Images are not distorted.• Improves peripheral vision. • Contact lenses should be used very carefully. • It affects the cornea if used improperly. Contact lens is preferred by those who do not want to use spectacles. It is very useful for sports persons, construction workers etc.MEMORY TIPSFigure of using contact lenses on eyes

282 Lens Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 283Instead of using thick powerful lenses in spectacles, we can use thin less powerful contact lenses.Instead of using thick powerful lenses in spectacles, we can use thin less powerful contact lenses because they stay closer to the cornea than the spectacles. FACTS WITH REASONSACTIVITY 7Make a 3D model of contact lenses. Laser eye surgery [ Laser-assisted in situ Keratomileusis (LASIK)]Laser eye surgery A person might have a defect of vision, which can be treated by spectacles and contact lenses. Laser eye surgery is another method of treating defects of vision. It is performed by reshaping the structure of the cornea. As we already know, short-sightedness occurs if the cornea is curved or raised conical. So, if we use an excimer laser to reshape that cornea and change it into a flat, the short-sightedness will be cured. Again, long-sightedness occurs if the cornea is flatter than usual, so we can cure it by reshaping the cornea into a bit more curved structure. We can use laser light to raise the middle part of the cornea to make it a bit curved or conical to cure long-sightedness. The treatment of a defect of vision by reshaping the cornea with the help of an excimer laser is called laser eye surgery. LASIK is the most common method of laser eye surgery. LASIK stands for Laser-Assisted in Situ Keratomileusis. Process of LASIKThe cornea is reshaped during the laser surgery to correct a defect of vision. A strong beam of ultraviolet rays is focused on the cornea. The beam cuts a flap out of the cornea. Then that flap is reshaped to treat the defect of vision. If the middle portion of the cornea is made flat by a laser beam, it will treat short-sightedness.If the middle portion of the cornea is made conical by a laser beam, it will treat long-sightedness. LASIK is a better solution for defects of vision than spectacles or contact lenses.LASIK is a better solution for defects of vision than spectacles or contact lenses because it is convenient, no need to wear lenses, no infection, does not break and guaranteed treatment. FACTS WITH REASONSACTIVITY 8Make a 3D model for laser eye surgery treatment procedures. Figure of performing laser eye surgeryThe excimer laser used during eye laser surgery is a special type of ultraviolet laser.MEMORY TIPS

282 Lens Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 283HOT SKILL HIGHER ORDER THINKING SKILL1. A convex lens can be used to start the fire from sunlight.The object (Sun) is at infinity. The parallel rays of light are falling in the convex mirror. So the parallel rays of light must bend at meet at focus. The image of the sun will made at focus. Thermal energy will be collected at focus. Then the fire will start. 2. Contact lenses are preferred by sports persons.Contact lenses are preferred by sports persons because they can be thin, have low power, have better vision, improve peripheral vision, do not fall etc.3. Lack of vitamin A can cause night blindness. Rhodopsin helps to see in dim light. Rhodopsin is made of protein and vitamin A. If vitamin A is lacking, rhodopsin will not be formed. So, the person cannot see in dim light. Hence, lack of vitamin A is the main cause of night blindness. 4. Which lens will make a virtual diminished image if an object is kept 4 cm away from both lenses where one has a focal length of 5 cm and another has a focal length of – 5 cm? Write with reason and then verify with a diagram. Since an object is 4 cm away from both lenses, it is at the location between the focus and the optical centre. The first lens has a positive focal length (5 cm). So, it is a convex lens. Here an object is between the focus and optical centre before the convex lens. So, it makes the virtual and magnified image. It can be verified by the given ray diagram:The second lens has a negative focal length (- 5cm). So, it is a concave lens. Here, an object is between the focus and the optical centre before the concave lens. So, it makes the virtual and diminished image. It can be verified by the ray diagram given below:5. A person cannot see an object at 25 cm but can see at 35 cm. What kind of lens should he use? Why?Usually, for a healthy eye, a near point, the least distance of distinct vision is 25 cm. That person is unable to see at 25 cm but able to see at 35. It means he is suffering from long-sightedness. That person's eye’s near point has shifted from 25 cm from the lens to 35 cm. Therefore, this person must use a convex lens because this person is suffering from long-sightedness. 6. Letters on the board become blurred when Radha moves far away from the whiteboard. Mention the defect of vision and its causes. She got spectacles to correct it. What type of spectacles is it? How does that lens help to correct this defect? Since letters become blurred when Radha moves far away from the whiteboard, she must be suffering from short-sightedness because she can see near objects but has difficulty seeing distant objects. The causes for short-sightedness are short focal length, elongated eyeball, the very steep curve of the cornea etc. The spectacles she got to correct short sight must be concave lenses because they can diverge parallel beams of light so that image can be formed at the retina. Short sight is caused when the parallel beam of light from distant objects meets ahead of the retina.

284 Lens Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 285When the concave lens is used as spectacles, the parallel beam of light from objects will be diverged a bit by the lens before reaching the eye lens. Then the eye lens will converge the light beam at the retina and she will be able to see the object. Concave lensImage ImageEye lensAffected by myopia Correction of myopia7. Differentiate between colour blindness and night blindness. The differences between colour blindness and night blindness are:SN Colour blindness SN Night blindness 1 The inability of an eye to detect the particular colour of objects is called colour blindness. 1 The inability of an eye to see properly in dim light is called night blindness.2 It is caused by the defect in cone cells. 2 It is caused by a defect in rod cells.8. Nancy was reading a book in her room. Meanwhile, she looked at a cat far away on a tree out of the window. How does the ciliary muscle function to change the shape of the lens and focal length so that she can easily see the cat far away? Discuss. Since Nancy was reading a book in her room, her eye lens must have been thicker. The books are closer to the eyes. A shorter focal length is enough to focus on a near object. However, the focal length of the eye lens must be increased now because she is looking at a cat far away on a tree. The focal length can be changed by changing the shape of the lens. The ciliary muscles help to change the shape of the eye lens.When the ciliary muscles relax, suspensory ligaments attached to the eye lens are pulled. The eye lens will be pulled from all sides. The lens will be stretched. It will become thinner. The thinner eye lens will have a longer focal length and it will help to see distant objects. 9. Eye donation is not popular in Nepal. Many people believe that doctors take an entire eye. Which part of an eye can be donated after death? What is it used for? Clarify the concept of eye donation to the people so that they would be encouraged for eye donation. Eye donation is not popular in Nepal yet because most people do not know about it or they have a misconception. The cornea of the eye can be donated after death. The harvested cornea is used for corneal transplantation to treat some defects of vision. Eye donation is the act of donating the cornea of the eye. The cornea is the outer transparent layer of the eye. A healthy cornea is harvested by doctors through minor surgery and stored for corneal transplantation. Some of them are also used for research. It is not the entire eye or lens or iris that is harvested or transplanted. 10. Pooja is sitting on the last bench. She has a normal healthy eye. She is copying notes from white board into her note book. How does the ciliary muscle help her to do so? Note book is nearer compared to the whiteboard. When she writes in a note notebook, the letters are nearer. So, the focal length of the eye must decrease. To decrease the focal length the lens must become thick. To make the lens thick, the ciliary muscle contract. So, when she looks at notes on the whiteboard, the focal length of the eye must increase. To increase the focal length the lens must become thin. To make the lens thin the ciliary muscles must relax.

284 Lens Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 2854 STEPS EXERCISE EXERCISESTEP11. Multiple Choice Questions a. A person cannot see any object closer than 30 cm from the eye. What defect of vision is he suffering from? i. long-sightedness ii. short-sightedness iii. eye cataract iv. corneal blindness b. Why can a lens bend the light?i. they are curved ii. they decrease the velocity of light iii. they increase the velocity of light iv. they are transparent c. A kid has difficulty seeing near objects in dim light. What is his defect of vision?i. short-sightedness ii. long-sightedness iii. night blindness iv. colour blindness d. What is the remedy for long-sightedness?i. vitamin A supplements ii concave lens iii. convex lens iv. green vegetables e. What is the procedure of cataract surgery?i. cut a hole in cornea → break clouded lens by ultrasound → remove broken lens → stitch intraocular lens → stitch cornea ii. cut a hole in lens → break clouded lens by ultrasound → remove broken lens → stitch intraocular lens → stitch lens iii. cut a hole in cornea → remove clouded lens → break the lens → stitch intraocular lens → stitch corneaiv. reshape the cornea → break clouded lens by ultrasound → remove broken lens → stitch intraocular lens → stitch lens f. Which of the following statements is true?i. real, inverted and diminished images only are formed by convex lensii. real, erect and magnified image is formed by the concave lensiii. the virtual, erect and magnified image is formed by the concave lensiv. virtual, erect and diminished images are only formed by the concave lensg. Contact lens does not distort the peripheral vision.i. because it is thinner than spectacles. ii. because it is closer to the eye than spectaclesiii. because it has lesser power than the spectacles iv. because it moves along with the cornea h. Study the given statement and arguments. Statement : When an object is placed between the focus and the optical centre, it makes a virtual and diminished image.Argument 1 : It is a concave lens used at the door peepholes. Argument 2 : It is a convex lens used at the door peephole. Which of the following is correct? i. Statement and argument 1 are correct but argument 2 is incorrect. ii. Statement and argument 1 are incorrect but argument 2 is correct.iii. Statement is incorrect but arguments 1 and 2 are correct.

286 Lens Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 287iv. Statement is correct but arguments 1 and 2 are incorrect. i. Who is suffering from keratoconus? Students Problems in eye Symptoms Cause Arya A Cannot see far Raised cornea Elle B Cannot see near Flat cornea Eleonor C Cannot see in dim light Lack of vitamin Ai. Arya and Elle ii. Eleonor iii. Elle iv. Arya j. What is the relation between the power of the lens and the focal length? i. power of the lens is reciprocal of the focal lengthii. power of the lens is directly proportional to the focal lengthiii. power of the lens is equal to the focal length iv. power of lens is the addition of focal lengtha. i b. ii c. iii d. iii e. i f. iv g. iv h. i i. iv j. i Ans of MCQSTEP22. Very short questions a. Which one of the following is long-sightedness?Ciliary muscle cannot contract enough Ciliary muscles cannot relax enough The lens cannot curve enough A BThe lens cannot be thin enough C Db. Shape of the cornea is changed to treat defects of vision. Write the name of the defect of vision A.Defect of vision A is treated. Defect of vision B is treated. raise the mid of cornea a bit to make it more curvedmake the mid of cornea a bit flat Cornea c. Junu was playing with her friends in class UKG. Her eye was accidentally scratched by the nails of her friend. What kind of difficulty can she suffer from abrasion of the cornea?d. Mention one application of such arrangement of the convex lens and the object.

286 Lens Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 287e. What is the meaning of the saying \"the power of a lens is 1 dioptre? f. Where an object should be kept in front of a convex lens so that the image becomes diminished, inverted and real? g. What is rhodopsin made of? h. If a protein crystal is accumulated in the lens of an eye and forms a grey spot, what disease is it?i. If laser surgery is used to make the mid part of the cornea flatter, which defect of vision is being treated? STEP33. Short questions a. Write any two differences between the concave lens and the convex lens. b. An object is placed between the optical centre and the focus of a convex lens. What are the features of the image formed?c. If a burning candle is placed at a distance of 4 cm from a convex lens having a focal length of 2 cm, draw a ray diagram to show the formation of the image. d. The corrected diagram of an eye with a defect of vision is shown. Draw a ray diagram of the defective eye when the given lens is not used. e. If a sharp tool scratches the cornea and liquid builds up at injury, it can blur vision. What is this kind of corneal injury called? What happens if the cornea becomes conical due to injury?f. Yugal studies in grade ten. He does not like to eat vegetables or fruits. Lately, he had difficulty seeing early in the morning and late at night. He bumps into furniture. What might have happened to him and why? What should he do to recover?g. Manish was reading a book in his room. Meanwhile, he looked far away from the window. How does the ciliary muscle function to change the shape of the lens and focal length so that he can easily see outside of the window right away and also help to look back at the book immediately? Discuss. h. Complete the ray diagram given alongside. Also, mention the nature of the image thus formed. 2F F' C F 2FAB

288 Lens Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 289STEP44. Long questions a. Study the given diagram and answer the following question.i. Which lens is used for book reading?ii. What is the position of the object?iii. Draw the ray diagram that describes this effect. b. A flashlight creates a parallel beam of light that can travel very far. Where should the source of light be placed behind the convex lens, so that the flashlight can produce a parallel beam? Show a ray diagram.c. Sharmila cannot see the text in her book but can read the text on the whiteboard. What are the causes of this defect? Draw the schematic diagram to represent this defect. She now wears certain spectacles to counter this defect of vision. Also, draw the diagram to show how did it correct this defect of vision.d. Study the news article written by Rajan Pokhrel in the Himalayan Times[Dr Sanduk Ruit wins top civilian award of BahrainDr. Sanduk Ruit is a noted Nepali ophthalmologist. He will receive the fifth edition of the Isa Award for Service to Humanity. It is a top civilian honour of the Kingdom of Bahrain. Dr Ruit is considered the god of sight. He succeeded in creating a new implantable lens that could be produced at a much lower price than it was before. It allows him to complete cataract surgery in significantly less time. Cataracts can be surgically removed through tiny cuts in as little time as five minutes. Cataract eyelens can be replaced with an inexpensive artificial lens.] What is the intraocular lens produced by Dr. Sanduk Ruit? Which part of the eye is replaced by the intraocular lens to treat eye cataracts? How does he treat eye cataracts? Describe briefly. What suggestions would he give to young people to avoid eye cataracts? 5. Numerical i. A person having a myopic eye uses a concave lens of focal length 10 cm. What is the power of the lens? [Ans: -10 D]ii. A convex lens has a focal length of 3 cm. A candle is placed at a distance of 4 cm in front of the lens. Draw a ray diagram to show the image formed by that lens. How much is the power of the lens? [Ans: 33.33 D]iii. A convex lens of power 10 D is used to burn a paper piece by converging the solar beam. What should be the distance between the lens and the paper piece to burn it? [Ans: 10 cm]iv. If a person wears spectacles with lenses of power – 4.5 D. What is the type of the corrective lens? What kind of defect of vision does he have ? Find the focal length of the lens used. [Ans: 22.22 cm]

288 Lens Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 289Key terms and terminologies of the unit1. Current: The rate of flow of electrons in a definite direction is called electric current. 2. Circuit: The electric circuit is a path made of conducting wire, source, loads and switches where current can flow.3. Open electric circuit: The electric circuit through which current cannot flow due to an interrupted path is called open electric circuit.4. Closed electric circuit: The electric circuit through which current can flow in an uninterrupted path is called closed electric circuit.5. Direct current (DC): The current that changes polarity at regular intervals is called alternating current. 6. Alternating current (AC): The current that cannot change polarity is called direct current. Sequence of Curriculum Issued by CDC Direct current and alternating current: introduction and average voltage and frequency used in mainline in Nepal Magnetic effect of electric current: introduction, magnetic field around a straight conductor and a solenoid and right-hand grip rule  Magnetic flux: introduction to magnetic flux; weak and strong magnetic flux Motor effect: introduction and application in daily life Electromagnetic induction: Introduction, Faraday's laws of electromagnetic induction  The working mechanism of AC generator and AC dynamo  Source of electricity: introduction, working mechanism and importance of hydroelectricity, thermal power plants and wind farms Transformer: introduction, construction, working principle and simple calculations based on working principle of transformer  Introduction and application of step-up and step-down transformer UNITElectricity and Magnetism11Estimated teaching periods Theory Practical9 3Gehendra Shumsher, born in Poush, 1928 BS, was a significant figure in Nepalese history, celebrated for his contributions to firearm design and military strategy. As the eldest son of Bir Shumsher, a prime minister of the Rana dynasty, Gehendra Shumsher's most notable achievement was the invention of the Gahendra Martini rifle, a pivotal addition to Nepal's military arsenal. His innovative advancements have left a lasting imprint on Nepal's military history, bolstering its defense capabilities. Gehendra Shumsher, About the Nepalese Scientist

290 e LectRicity and MagnetisM Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 2917. Fluorescent lamp: A discharge lamp coated with fluorescent material from inside is called fluorescent lamp.8. Magnetic effect: The physical process in which a magnetic field is produced around the conductor when current passes through it is called the magnetic effect of current.9. Solenoid: A solenoid is a cylindrical coil of wire.10. Electromagnet: A temporary magnet made by passing electric current around a magnetic substance through an insulated wire is called electromagnet. 11. Electromagnetic induction: The process of inducing current in a closed coil due the change in magnetic flux because of the relative motion between the magnet and the coil is called electromagnetic induction.12. Bicycle dynamo: A bicycle dynamo is a very simple dynamo in which a magnet is made to spin round near a fixed coil.13. Generator: An electric generator is a device that converts mechanical energy into electric energy on the basis of electromagnetic induction.14. Motor effect: When a freely suspended current carrying conductor is kept in a magnetic field then it comes in motion. This effect is called motor effect. 15. Electric motor: An electric load that converts electricity into kinetic energy (mechanical energy) is called an electric motor.16. Transformer: A transformer is a device that converts high voltage AC to low voltage AC and vice versa. 17. Step-up transformer: A transformer which is used to increase the magnitude of alternating current AC is called a step-up transformer. 18. Step-down transformer: A transformer which is used to decrease the magnitude of alternating current AC is called a step-down transformer. IntroductionElectricity is an important source of energy in modern world. It is a convenient form of energy like heat, light, etc. Electricity is used in our homes, offices, industries, laboratories and many other places for lighting and heating purposes. It is used in our homes for lighting tubes and bulbs, operating fans, TV, radio, charging mobile, using heater, etc. While using electricity, the loads are connected in series or parallel in an electric circuit. Such loads convert electrical energy into other forms of energy. CurrentElectromotive force (emf) is required to flow electrons in a circuit. This flow of electrons in a circuit results into electric current. Thus, the rate of flow of electrons in a definite direction is called electric current. For a small current, the source of emf in a circuit is battery. But for the vast majority of electric devices hydroelectricity is the main source of energy. Electricity is generated by electromagnetic induction in the hydro-power station. Transformers are used to increase or decrease the emf of the electricity. In this unit, we will discuss the effects of electric current, electromagnetic induction, principle of generator, electric motor and transformer.

290 e LectRicity and MagnetisM Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 291Electric CircuitThe electric circuit is a path made of conducting wire, source, loads and switches where current can flow. There are two types of electric circuits. They are an open circuit and a close circuit. The electric circuit in which the switch is off, wires are broken etc. is called an open circuit. The loads do not work in this circuit. The electric circuit in which the switch is on, wires are connected and current can flow is called a closed circuit. Load can work in this circuit. Direct Current (DC) and Alternating Current (AC)Direct CurrentElectric current is a flow of electrons through a conductor. If the current always flows in the same direction in a conductor, it is called direct current (DC). A cell has fixed positive and negative electrodes. The current flowing from a cell is DC. Dry cells, photo cells, solar panel, etc. are the sources of DC. Alternating CurrentIn alternating current, the current changes direction and flows forward and backward. The current whose direction changes periodically is called an alternating current (AC). AC generator, bicycle dynamo, etc. are the sources of AC. In each cycle of AC, the current changes its direction two times. The frequency of AC used in our domestic circuit is 50 Hz, which means the current changes its direction 100 times in one second. Average voltage and frequency used in the mainline of NepalVoltage supplied for domestic use and the frequency of alternating can be different in different places of the world. In Nepal alternating current of 220 volts to 240 volts and a frequency of 50 Hz is supplied for domestic use. The total number of complete cycles made per second is called frequency. Its SI unit is Hertz (Hz). One hertz frequency means alternating current made one complete cycle in one second. It means it changed direction 2 times. The frequency of alternating current at Kathmandu in Nepal is 50 Hz. It means the current distribution at Kathmandu in Nepal makes 50 complete cycles in one second. It means the direction of AC changes 100 times per second. The armature and turbine of the generator have to make 3000 rotations per minute to produce a 50 Hz frequency. FACTS WITH REASONSThe frequency of AC used in our domestic circuit is 50 Hz. What does it mean?The frequency of AC used in our domestic circuit is 50 Hz. It means that 50 cycles are made by the alternating current in one second.Switch CellCopper wireBulbElectric circuitCurrent time graph of DCTime CurrentCurrent time graphA current-time graph is a graph that shows how the current flowing in an electric circuit changes over time. MEMORY TIPSCurrent time graph of AC CurrentTimeRectifier converts AC to DC to run mobile, computer, etc. It is also called diode. It is present in inverter. MEMORY TIPS

292 e LectRicity and MagnetisM Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 293Differences between AC and DC.S.N AC S.N. DC1. Alternating current has a frequency. It changes polarity. 1. Direct current has zero frequency. It does not change polarity.2. It is produced by AC generator, dynamo, etc.2. It is produced by cells, battery, DC generator, etc.3. The magnetic field produced by AC changes its poles alternately.3. The magnetic field produced by DC has fixed poles. 4. The voltage of the AC can be changed by the transformer.4. The voltage of the DC cannot be changed by the transformer. Magnetic Effect of Current: Relation between Electricity and MagnetismThe relation between electricity and magnetism was not known until Oersted's discovery. Electricity and magnetism were considered as two separate fields of science. A current carrying wire behaves like a magnet and creates a magnetic field around it. This magnetic field remains around the wire until there is electric current in the wire. When current is stopped, the magnetic property associated with the wire also vanishes. Thus, when current is passed through a wire, it produces a magnetic field around it. This effect of an electric current is called magnetic effect of current. Magnetic field around a straight current carrying wireA current-carrying wire acts like a magnet and makes a magnetic field around it. This magnetic field remains around the wire until there is an electric current in the wire. When the current stops going through a wire, the magnetic property of the wire also goes away.Direction of the magnetic field around the straight current-carrying conductorThe direction of the magnetic field around the straight current-carrying conductor is described by Maxwell's right-hand thumb rule (Maxwell’s corkscrew rule). According to Maxwell's right-hand thumb rule, if the thumb represents the direction of electric current in a straight conductor, then the curling fingers represent the direction of circular magnetic fields and vice versa. If the current is moving up then the direction of the magnetic field will be anticlockwise. If the current is moving down, the direction of the magnetic field will be clockwise. ACTIVITY 11. Take a cardboard and pass a straight conducting wire through its center. 2. Sprinkle iron filling on the cardboard.3. Connect the two ends of the wire with a battery.4. Do you observe the iron filling arranged in circle around the wire?The magnetic field around a straight current carrying conductor is in circles round the wire. The tangent that is drawn on the circle gives the direction of magnetic field at that point. If the direction of current is reversed the magnetic field will still exist in circles around the wire, but its direction is reversed.

292 e LectRicity and MagnetisM Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 293Magnetic field around a solenoidA solenoid is a cylindrical coil of wire. The working principle of the solenoid is the magnetic effect of current. It means when the current flows through the insulated coils of the solenoid a magnetic field is generated around it. Solenoid is used to make electromagnets because it makes temporary magnets. A temporary magnet which is made by using an electric current is called an electromagnet. Solenoids are used to make transformers, generators and dynamos, electric bells and magnetic parts of cranes and MRI machines. ACTIVITY 21. Make a solenoid by closely winding 20-25 turns of insulated copper wire around a cylindrical object like a glue stick. 2. Remove the stick and connect the two terminals of the wire to a cell. 3. Bring a magnetic compass needle close to the solenoid. Does the needle show deflection? In which direction does the needle deflect when the polarity of the cell is changed?The magnetic field produced by a solenoid is similar to the magnetic field produced by a bar magnet. In solenoid, the magnetic lines of force pass through it and return to the other end. The parallel magnetic lines of force inside the solenoid indicate that the strength of magnetic field is same at all the points inside the solenoid.A freely suspended current-carrying solenoid rests in a north-south direction because it acts like a bar magnet. Factors affecting the strength of the magnetic field produced by solenoidThe magnitude of the magnetic field produced by the solenoid depends mainly upon the voltage (emf) of the current supplied, the number of turnings in the solenoid and the nature of the iron core.We can increase the strength of the magnetic field of the solenoid by increasing the voltage of the current, using a good quality soft iron core and increasing the number of turns of the solenoid. Direction of the magnetic field around the solenoidThe direction of the magnetic field around the solenoid can be described by Maxwell’s Right-hand grip rule. According to Maxwell’s Right-hand grip rule, Electric current passes through a solenoid resulting in a magnetic field. When we wrap our right hand around the solenoid with our fingers in the direction of the current, our stretched thumb points in the direction of the magnetic north pole.Magnetic FluxThe magnet is a substance which has a north pole and south pole and can attract iron. The magnet has a magnetic force that can attract irons. Those magnetic forces move from south to north inside the magnet and from north to south outside the magnet. A unit north pole comes

294 e LectRicity and MagnetisM Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 295out from the magnet travels in a curve and reaches the south pole. The imaginary path in which the unit north pole travels from the north pole and enters the south pole is called magnetic lines of force. If any metal conductor is placed inside the magnetic fields, some magnetic lines of force pass from the conductor. The total number of magnetic lines of force that pass through the given closed area of the conductor is called magnetic flux. The SI unit of magnetic flux is Weber (Wb). It is named after the German physicist Wilhelm Eduard Weber. The ends of magnets have poles. The magnetic lines of forces are more crowded near the poles than the mid. Therefore, there is more flux density at the poles and less flux density at the middle part of the magnet. Hence, the magnetic field is stronger at the poles and weaker in the middle. The magnetic field is weaker as we move far away from the bar magnet because of the decrease in the number of magnetic lines of force, magnetic flux and flux density. The magnetic flux is represented by the Greek letter phi (Φ).Motor Effect Oersted discovered that electric current produces magnetic field around the current carrying conductor. It is called magnetic effect of electric current. When this current carrying conductor is kept in another magnetic field, then these two magnetic fields interact. The forces between the two magnetic fields can move wire. Thus, when a freely suspended current carrying conductor is kept in a magnetic field, it comes in motion. This effect is called motor effect.FACTS WITH REASONSIf we supply a higher voltage of current, the motor rotates faster.If we supply a higher voltage of current, the motor rotates faster because it produces a powerful magnetic field around the armature. The powerful magnetic fields repel each other faster, so the motor rotates faster. ACTIVITY 3Use an insulated copper coil, magnets and a battery to show the motor effect. ACTIVITY 4Take a powerful horseshoe magnet, a battery and a copper coil. Set them as shown in the diagram and pass electricity. What happens to the copper wire? What is this effect called? Discuss. Fig: Magnetic fluxElectric motor contains solenoids in them.MEMORY TIPSMotionconducting wirecurrentMagnetic lines s of forcenMotor effect

294 e LectRicity and MagnetisM Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 295Electric MotorA device which converts electrical energy into kinetic energy (mechanical energy) is called an electric motor. An electric motor is based on the principle of motor effect. A permanent magnet is used in it. Structure and working mechanism of an electric motorThe magnetic field and the armature that can rotate are two major components of the electric motor. The container contains powerful magnets. An armature is fixed inside the magnetic field. Then the armature is connected to the source of electric current. If it is a DC motor, it is connected to a DC source. If it is an AC motor it is connected to an AC source. Whenever the current is passed into the armature, a magnetic field is produced. Now there are two magnetic fields inside the device. They repel each other to produce motion. Once the coil begins to spin, the two magnetic fields interact with each other. During one half of the rotation of the coil, there is attractive force between the coil and the permanent magnet. In next half, this force changes into repulsive force. In this way, the coil continuously rotates and converts electrical energy into mechanical energy.Factors affecting the turning force on a motorIn a motor, the turning force on its coil depends on:i. the strength of the magnetic fieldii. the amount of the current suppliediii. the number of turns in the coilApplications of electric motorElectric motors are used in many ways in our daily life.i. An electric fan rotates due to the motor present in it.ii. Electronic devices like, computer, DVD players, etc. have motors in them.iii. A water lifting pump contains a motor.iv. Saw mills, rice mills, etc. have powerful motors.v. Electric motors are now used to make electric bikes and electric cars.Electromagnetic InductionMoving a conductor in a magnetic field is the basis for a simple electric generator. When a conductor moves in a magnetic field, the magnetic lines of force are intersected by the conductor. As a result, there occurs change in magnetic flux. This change causes current to flow through the conductor. Thus, the process of inducing current in a closed coil due to the relative motion between the magnet and the coil is called electromagnetic induction.Electric motorElectric motorElectromagnetic induction

296 e LectRicity and MagnetisM Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 297Fleming's right hand ruleFleming's right hand rule states that when the thumb, fore finger and central finger of the right hand are adjusted at right angles to each other in such a way that the thumb points in the direction of motion of conductor, fore-finger points in the direction of magnetic field, then the central finger points in the direction of the induced current in the conductor.MEMORY TIPSACTIVITY 51. Make a coil by winding about 50 turns of thin insulated copper wire round a match box.2. Connect the two terminals of the coil with a galvanometer.3. Move a pole of a bar magnet in and out of the coil and observe the deflection of needle in galvanometer.4. Observe how the magnitude of the induced current depends on (i) the speed of the relative motion,(ii) the strength of the magnet, (iii) the number of turns on the coil, etc.Conclusion:i. When a magnet is moved in and out quickly into a coil, a large current gets induced. ii. When a powerful magnet is used, the magnitude of the induced current increases. This is because a powerful magnet has more lines of force between its poles. So, when a wire moves through its field it cuts the magnetic lines of force more often.iii. When the number of turns in a coil increases, the induced emf also increases.GalvanometerTurns of wireMatch boxMagnetSElectromagnetic induction was discovered in 1831 by Michael Faraday. The Voltaic cells were used as the main source of electricity in the nineteenth century starting from 1800 AD. He was the first to observe how the magnitude of induced emf depends upon the relative motion between the coil and magnet. Faraday observed the deflection of galvanometer needle in opposite direction when the direction of motion of conductor is changed. This is due to the change in direction of induced current while changing the direction of motion of the conductor in between two magnetic poles. MEMORY TIPSIn the above activity, if the magnet is moved away from the coil with the south pole pointing towards the coil, then it produces deflection in the opposite direction as compared to the case when the magnet is moved towards the coil.Faraday's Laws of Electromagnetic InductionFaraday’s laws of electromagnetic induction can be summarised in three points. They are:i. Whenever there is a change in magnetic flux, emf is always induced in the linked conductor. ii. The magnitude of the induced emf is directly proportional to the rate of change of magnetic flux.iii. The induced emf will last as long as there is a change in magnetic flux.