Modern Concept Science and Technology 10 New

Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 247 rarer medium, air, and then reach our eyes. So, when the light is passing from water to the air it bends away from the normal. As a result, the image of the fish is formed slightly above the real depth. So, fish is seen at apparent depth instead of real depth. Hence, it is hard for Elle to pinpoint the location of the fish and fails to hit. 7. Water is a rarer medium compared to glass but denser compared to the air. Give reason. An optical medium where the velocity of light is more is a rare medium and that in which the velocity of light is less is a denser medium. The velocity of light in air is 3 lakhs kilometres per second. The velocity of light in water is 2.2 lakhs km/s. The velocity of light in glass is 2 lakhs km/s. Here, the velocity of light in water is slower than the velocity of light in air. So, water is a denser medium compared to air. But the velocity of light in water is greater than the velocity of light in glass. Hence, water is a rarer medium compared to glass. 8. 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. 9. Why do stars twinkle when observed from earth but not when observed from international space stations? The stars are far away and appear as a point source of light. It covers a too small area in the night sky. When their light has to pass through the atmosphere, they travel through multiple layers of air of varying densities and temperatures. So, starlights suffer multiple refractions of light and their images are formed in multiple places in the sky but within a small overlapping region. Hence star twinkle. The stars do not twinkle from the international space station because there is no atmosphere around ISS. Since there is no atmosphere light won’t refract. Star will be observed at its original place. 10. Mukesh once cut open a fibre net on his internet connection. He saw no wire but a thin flexible, hollow and transparent tube made of either plastic or silica. What is it? What is its function? On which principle does it work? How does it work? Why is it preferred over the copper wire? When the fibre net was cut, Mukesh did not find any wire but a thin, flexible, hollow and transparent tube. This tube is called optical fibre. It is a modified version of a light pipe. Its function is to transfer electromagnetic waves and data along with it from one corner of the light pipe to another end. It works on the principle of total internal reflection of the light. Working mechanism When an electromagnetic wave enters the light pipe at an incident angle greater than the critical angle, it suffers total internal reflection. Numerous total internal reflections occur in the light pipe and the electromagnetic waves eventually reach another end of the light pipe. Optical fibres are preferred over copper wire because they can transfer light rays by multiple total internal reflections faster without any loss. 3 STEPS 3 STEPS EXERCISE EXERCISE STEP1 1. Select the best answers from the given alternatives. 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

248 ligHt Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur b. Why does 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 diamond sparkle a lot? i. it is transparent ii. its critical angle is 24 degrees iii. it is hard iv. it is made of glass. d. 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 e. What is the cause of the dispersal of light? i. density of the medium. ii. refraction of light iii. different velocities of the different rays of light iv. different colours of the light f. What is the property of a denser medium? i. it has a higher density. ii. light has lesser speed in it iii. light bends away from the normal iv. light has a higher speed in it 2. Define the following terms with required examples. a. Light b. Rarer medium c. Refraction of light d. Refractive index e. Real depth f. Critical angle g. Mirage h. Dispersion i. Optical medium j. Denser medium k. Cause of refraction l. Cause of dispersion m. Apparent depth n. Total internal reflection o. Light pipe p. Absolute refractive index 3. Answer the following questions in very short. a. How much is the speed of light in vacuum, water and glass? b. What do you understand by 'rectilinear propagation of light'? c. State the principle of reversibility of light. d. What is the angle of refraction when a ray of light is incident normally on the water surface? e. State Snell's law of refraction of light. f. In which medium, among water, glass, and diamond, does i. light travel with the maximum speed? ii. light travel with the minimum speed? g. What is fibre optics? h. Write down the use of fibre optics. i. Write down the use of the light pipe. j. During the refraction of light through glass slab, which angle is equal to the angle of emergent?

Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 249 STEP2 4. Give reasons. a. A ray of light travelling from a rarer medium to a denser medium bends towards the normal. b. A pond appears to be shallower than it really is when viewed obliquely. c. A pencil dipped in water appears bent. d. The apparent depth of the water in a pond is less than the real depth. e. Stars twinkle on a clear night. f. Diamond sparkles. g. If we put a coin in an empty glass and lower our eyes so that it just disappears from vision, it reappears if water is added to the glass. h. A glass slab was put over an old wooden antique table to protect it from damage. The letters carved on the wood appeared closer and bigger due to the glass. Why? i. Prism can disperse the sunlight. j. When an erect prism is aligned with an inverted prism, the combination cannot disperse the light. 5. Differentiate between the following. a. Optically rarer medium and optically denser medium b. Real depth and apparent depth c. Reflection of light and total internal reflection d. Refraction and dispersion 6. Answer the following questions in short. a. State the laws of refraction of light. b. How are the angle of incidence and angle of refraction related to each other when a ray of light travels from: i. a rarer to a denser medium. ii. a denser to a rarer medium. c. What does the refractive index of a medium indicate? Explain with an example. d. Under what conditions does Total Internal Reflection (T.I.R.) take place? e. Explain any two effects of total internal reflection. f. With respect to air, the alcohol and benzene have refractive indices of 1.36 and 1.50, respectively. Which is optically denser of the two? g. You have been given 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 and minimum, respectively? h. On which factor does the refractive index depend? i. What is a mirage? Explain its formation. j. When white light passes through a prism, which colour i. deviates the least ii. deviates the most. Also, write the reason. k. How can you show that the white light consists of seven colours? Discuss the formation of the rainbow.

250 ligHt Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur l. Draw the diagram i. to show the complete path of the light ray that travels through a rectangular glass slab. ii. to show the refraction of light when a ray of light travels from a. air to glass b. glass to air iii. to show the apparent position and real position of a coin in a beaker filled with water. iv to show why a stick obliquely immersed in water appears to be bent. v. to show the critical angle of glass. vi. to show dispersion of light through a prism. vii. to show the bending of light when an incident ray passes from water to air where the angle of incidence is 42°. m. Discuss the role of a medium in the bending of light when light propagates through it. n. Explain any two effects of refraction of light experienced by you. o. How does total internal reflection of light help in keyhole surgery? STEP3 7. Answer the following long questions. a. A ray of light is incident normally on various surfaces of glass. Complete the path of the light ray. Air Air 450 420 Glass (a) (b) (c) (d) Glass 900 450 b. A beam of white light passing through a glass prism is shown in the given figure. i) Which phenomenon of light is shown in the given figure? ii) Name each of the seven colours in order. iii) Which colour deviates maximum during the dispersion of light through the prism? Write with reason. iv) Write the order of the wavelength of the colours at the top and the bottom on the screen. c. The speed of light in a medium 'M' is 0.75 × 108 m/s and that in a vacuum is 3 × 108 m/s. Find the refractive index of 'M'. [ Ans: 4] d. 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] e. 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]

Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 251 Key terms and terminologies of the unit 1. 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) UNIT Lens 10.2 Estimated teaching periods Theory Practical 6 2 Euclid is well known as the father of geometry for his concept of Euclidean geometry. He was born in ancient Greek in 325 BC and died in 265 BC. He was a mathematician and logician. He has developed Euclidean geometry, Euclidean algorithm and Euclid’s Theorem. Besides that, he is well known for geometrical optics. It describes light propagation in terms of rays and beams. It helps to explain reflection of light from plane mirrors and spherical mirrors. It is also used to explain the refraction of light through glass slabs, concave lenses and convex lenses. Euclid About the Scientist

252 lenS Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur 10. 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: Instruments that are used to produce an image of an object with the help of a lens or lenses 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 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. 24. Myopia: Myopia is the defect of vision in which a person can see near objects clearly but cannot see distant objects. 25. Hypermetropia: Hypermetropia is the defect of vision in which a person can see distant objects clearly but cannot see nearby objects. 26. Corneal injury: The damage to the cornea due to physical causes, chemical causes or diseases that affect the vision of 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 certain colours in the usual way 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 is called eye cataract. 31. Contact lenses: The contact lenses are transparent curved and soft lenses that are put on the eyes directly.

Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 253 Introduction We 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. Lens A 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 Lenses If 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 Lenses Lenses 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 lens i. Convex Lens A 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

254 lenS Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Types 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 convex MEMORY TIPS ACTIVITY 1 1. 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 lens It 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 has a real focus, why? The refracted rays from a convex lens actually meet at a point. So, a convex lens has a real focus. It is dangerous to look through a convex lens at the sun or a bright light, why? A convex lens converges the parallel rays of light at a point increasing temperature at that point. If we look through a convex lens at the sun in a bright sunny day, it can damage our eyes. So, it is dangerous to look through a convex lens at the sun or a bright light. FACTS WITH REASONS ii. Concave Lens A 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

Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 255 A concave lens is called a diverging lens. Why? A concave lens is also known as diverging lens because it diverges the parallel rays of light after refraction. FACTS WITH REASONS Types 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 concave MEMORY TIPS A concave lens is made from the group of prisms: diverging action of a concave lens It 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 REASONS Terminology Related To Lens a. Center of curvature The 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. Aperture The maximum portion of the spherical surfaces from which refraction takes place is called aperture of the lens. diverging lens

256 lenS Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Center of curvature (C1 ) Center of curvature (C1 ) 2F F F O 2F X Y X Y Principal axis 2F F F 2F Center of curvature (C2 ) Center of curvature (C2 ) O A A B B Concave lens Convex lens R R R R Aperture of convex lens Aperture of conave lens c. Radius of curvature The 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 center The 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 axis The line joining both the centers of curvature of a lens is called the principal axis. In the given figure, C1 C2 is the principal axis. f. Principal focus In 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) F Parallel rays of light Principal axis F' (Focus) (Focus) (Focal length) (a) (b) F f (Focal length) Focus Principal axis Parallel rays of light Concave lens or diverging lens Convex lens or converging lens f O F O Principal focus of convex lens and concave lens g. Focal length The 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 = R 2 . MEMORY TIPS

Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 257 ACTIVITY 2 To determine the approximate focal length of a convex lens 1. 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 diagrams Rule 1 The ray of light which is parallel to the principal axis of a convex lens, passes through its focus after refraction through the lens. In case of a concave lens, the light ray appears to diverge from its focus after refraction. F O F O Convex lens Concave lens Rule 2 A ray of light passing through the focus of a lens becomes parallel to its principal axis after refraction through the lens. F O F O Convex lens Concave lens Rule 3 A ray of light passing through the optical center of a lens does not deviate after refraction through the lens.

258 lenS Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur O O Convex lens Concave lens Image and Its Types An 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 Lens A 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 3 1. 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 Candle Screen with candle For 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 TIPS Convex lenses are very similar to concave mirror in their image forming properties. MEMORY TIPS A convex lens can form both real and virtual images depending upon the distance between object and lens. MEMORY TIPS

Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 259 1. When object is at infinity Characteristics of the image i. Real ii. Inverted iii. Diminished to a point iv. Formed at focus 'F' on the other side of the lens Application : This type of image formation is used in burning glasses, astronomical telescope and camera 2. When object is beyond 2F Characteristics of the image i. Real ii. Inverted iii. Diminished iv. Formed between F and 2F on the other side of the lens Application : This type of image formation is used in a photographic camera. 3. When object is at 2F Characteristics of the image i. Real ii. Inverted iii. Equal to the size of the object iv. Formed at 2F on the other side of the lens Application : This type of image formation is used in a terrestrial telescope. 4. When the object is between F and 2F Characteristics of the image i. Real ii. Inverted iii. Magnified iv. Formed beyond 2F on the other side of lens Application : This type of image formation is used in film and slide projectors. 5. When the object is at the focus F Characteristics of the image i. Real ii. Inverted iii. Highly magnified F B' D A' Image (Real, inverted) O F' B A Parallel rays from top point of a distant object 2f Image (Real, inverted) Convex lens B 2F' F' A O F A' B' 2F 2f F O F 2F A B B' A' 2F Image (Real, inverted) Object F' C F 2F A' B' 2F Object Image (Real, inverted and magnified) A D B X Image at infinity Rays become parallel Object Convex lens F A O F Y B f

260 lenS Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur iv. Formed at infinity, on the other side of the lens. Applications : This type of image formation is used to make searchlight and spotlights in theaters. A convex lens used in powerful lamp produces a powerful parallel beam of light. 6. When the object is between the optical center (O) and focus (F ) Characteristics of the image i. Virtual ii. Erect iii. Enlarged (or magnified) iv. The image is formed on the same side of the object (beyond the object). Applications : This type of image formation is used in a magnifying glass or hand lens. It can be used in the following cases. i. Palmists use it to study the lines of the palm. ii. It is used as a reading glass. Summary of the Images Formed by a Convex Lens Location of the object Location of the image Size of the image Nature of the image 1. At infinity At focus (F) Highly diminished Real and inverted 2. Beyond 2F Between F and 2F Diminished Real and inverted 3. At 2 F At 2 F Same size as the object Real and inverted 4. Between F and 2F Beyond 2F Magnified Real and inverted 5. At F At infinity Highly magnified Real and inverted 6. Between F and O Image formed on the same side of the object Magnified Virtual and erect Image Formed by a Concave Lens Concave 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 infinity Characteristics of the image i. Virtual. ii. Erect. iii. Highly diminished to a point size iv. Formed at focus on the same side of the object Image (virtual, erect and enlarged) Convex lens O F B A' F A B' Image Object The image formed by a concave lens is upright, virtual and diminished for any object regardless of the distance between the object and lens. MEMORY TIPS F O F

Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 261 Application : This type of image formation is used as an eye lens in Galileo Telescope. 2. When the object is anywhere between the optical center and infinity Characteristics of the image i. Virtual ii. Erect iii. Diminished iv. Formed between the optical center and principal focus on the same side of the object Applications : This type of image formation is used for the correction of the short sightedness. Differences between convex lens and concave lens S.N. Convex lens S.N. Concave lens 1. A convex lens is thick in the middle but thin at the edges. 1. A concave lens is thin in the middle but thick at the edges. 2. It converges the parallel rays of light at a point in the principal axis. 2. It diverges the parallel rays of light from a point in the principal axis. 3. It has a real focus. 3. It has a virtual focus. 4. The power of a convex lens is positive. 4. The power of a concave lens is negative. 5. It can form real or virtual image depending on the position of the object. 5. It always forms a virtual image. Power of a lens A convex lens converges the parallel rays of light falling on it, whereas a concave lens diverges. Thus, the 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) = 1 focal length (in metre) = 1 f A 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 lens X Eye Center line of the lens (Virtual, erect and diminished) Object Image F B' A' B C Y A Concave lenses are very similar to convex mirror in their image forming properties. MEMORY TIPS A lens of short focal length has more power, whereas a lens of long focal length has less power. MEMORY TIPS

262 lenS Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur The power of a lens is measured as reciprocal of its focal length, why? A thick lens with short focal length has more ability to conserve or diverge the light rays. But the thin lens with longer focal length has less ability to conserve or diverge the light rays. It means that the power of a lens with short focal length is more and vice- versa. So, the power of a lens is measured as reciprocal of its focal length. FACTS WITH REASONS Unit of the power of a lens If 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 power The power of a lens whose focal length is one meter is called one dioptre. Solved Numerical 10.2.1 A 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 = –50 100 m = – 0.5 m Now, the power of a lens, P = 1 – 0.5 = –2 D Solved Numerical 10.2.2 A 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 cm If 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 cm or, f = 25cm = 25 100 m = 0.25 m Now, power of a lens, P = 1 0.25 = 4 D Thus, the power of the convex lens is +4 D. Magnification When an image is longer than the object, we say it is a magnified image. Similarly, when the image is smaller than the object, we say it is diminished. Thus, the ratio of the height of the The power of a lens can be measured directly by using an instrument called dioptometer. Opticians use it to measure the power of the lenses of spectacles. MEMORY TIPS

Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 263 image formed by a lens to the height of the object is called magnification of the lens. Mathematically, Magnification (m) = Height of the image (I) Height of the object (O) = I O Relation Between Magnification (m) and the Size of the Image (I) i. If height of image (I) = height of object (O), then m = 1. It means no change in the size of the image ii. If height of image (I) > height of object (O), then m >1. It means that image is bigger than the object. iii. If height of image (I) < height of object (O), then m <1. It means that image is smaller than the object. Magnification of a lens is five. What does it mean? Magnification of a lens is five. It means that the height of image (I) is five times longer than the height of the object. FACTS WITH REASONS To obtain a relation involving object distance (u) and image distance (v) In the given figure, Height of the object (AB) = O Height of the image (DC) = I Object distance (OA) = u Image distance (OD) = v In triangles AOB and COD: S.N. Statement Reason i. AOB = COD Vertically opposite angles ii. OAB = ODC Both of them are right angles iii. ABO = DCO If two angles of a triangle are equal to two angles of another triangle, then the third pair of angles is also equal. The triangles AOB and COD are similar. In case of the two similar triangles, the ratio of corresponding sides is equal. i.e. DC AB = OD OA I O = v u = m Magnification (m) = Height of the image (I) Height of the object (O) = Image distance (v) Object distance (u) Object Convex lens 2F F F 2F B O C D A u v

264 lenS Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur So, the magnification produced by a lens is also defined as the ratio of the image distance (v) to the object distance (u). Magnification has no unit, as it is a ratio of two distances. Solved Numerical 10.2.3 A convex lens forms an image of the object at a distance of 12 cm when the object is at 6 cm. Find the magnification of the image. Solution: Given, Object distance (u) = 6 cm Image distance (v) = 12 cm Now, magnification (m) = v u = 12 6 = 2 Solved Numerical 10.2.4 An object of height 2 cm is placed at a distance double of the focal length of a convex lens. Find the size of the image and its magnification. Solution: Given, Size of the object (O) = 2 cm When the object is placed at a distance double of the focal length of a convex lens, then Size of object the object (O) = Size of the image (I) = 2 cm Now, magnification (M) = I O = 2 2 = 1 Solved Numerical 10.2.5 A 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. The ray diagram of the image formed is shown in the given figure. Nature of the image : Real; Inverted; Magnified Formed beyond 2F on the other side of lens. Solution: Given, Focal length (f) = 2 cm = 0.02m Power of lens (P) = 1 f = 1 0.02 = 50 D Application: This type of image formation is used in film and slide projectors. The 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, 1 f = 1 u + 1 v Here, f = focal length, v= image distance and u = object distance. This formula can be applied both in convex and concave lenses. MEMORY TIPS F' C F 2F A' B' 2F Object Image (Real, inverted and magnified) A D B Human eye is an optical instrument similar to a photographic camera. MEMORY TIPS

Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 265 Optical Instruments Camera, 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 Eye Eyes 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 : Sclerotic is the outermost covering of the eye. 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 : It is the layer of tissues present inside the sclera which supplies blood to the eye. It also contains a black colouring which reduces reflection of light within the eye. 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 transparent 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. Cornea 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 TIPS Pupil Iris Distance vision Near vision

266 lenS Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur 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. Iris Sclerotic Retina Choroid Optic nerve Suspensory ligament Ciliary muscles Cornea Aqueous humour Structure of human eye Range of Vision and Accommodation i) Far Point The 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 Point The nearest position at which the eye can see tiny objects clearly without any strain is called the near point of the eye. For a normal eye, the near point lies between 25 cm and 35 cm. Generally, the near point is at 25 cm. iii) Least distance of distinct vision The 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 vision The distance between the near point and the far point is called the range of vision.

Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 267 v) Power of accommodation Eye 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 relaxed Image Parallel rays from a distant object Object at infinity Here eye lens has large focal length but small converging power Eye lens with large focal length but small converging power Here eye lens has short focal length but large converging power Image Ciliary muscles are tense Diverging rays from nearby object 25 cm O Eye lens with small focal length but large converging power We cannot see our surroundings clearly when we enter a dark room from a bright sunshine, but our vision improves after some time. Why? The size of the pupil of our eye is small in the bright light. So, when we enter the dark room, very little light enters into our eyes through the small pupil. As a result, we cannot see our surrounding clearly. After some time, the pupil of our eye expands and allows more light to pass through it. Therefore, we can see our surrounding properly. We cannot see object placed at a distance of less than 25cm. Why? The near point of the normal eye is 25 cm. Up to this distance, the eye shows its power of accommodation. Less than this distance, the ciliary muscles cannot make the eye lens bulge more. Therefore, we cannot see objects placed at a distance of less than 25cm. FACTS WITH REASONS Corneal Injury The cornea is the transparent front bulging part of the sclera. The curve of the cornea helps eyes to focus on the object by refracting the light rays. The damage to the cornea due to physical causes, chemical causes or diseases that affect the vision of eye is called corneal injury. This is because they can leave scars or discolourations. The scars and discolourations can block the light or distort the light that enters the eyes. As a result, eyes will be more sensitive to light, tears flow a lot and vision will blur. Figure to show corneal injury The cornea can be scratched by contact with dust, dirt, sand, wood shavings, plant matter, metal particles, contact lenses or even the edge of a piece of paper. MEMORY TIPS A person having a myopic eye can see nearby objects clearly but cannot see the distant objects clearly. The far point of an eye suffering from myopia is less than infinity. MEMORY TIPS

268 lenS Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Corneal abrasion can be caused by various reasons. Scratches, infections such as trachoma, viral infection, bacterial infection and fungal infection can damage the cornea. It can cause a corneal ulcer called keratitis. If a cornea loses its hemispherical shape to a conical, a person suffers from short sight. This defect of vision is called keratoconus. If left untreated it can cause permanent blindness. Sometimes, fluid builds up between the layers of the cornea. Light will not be refracted properly. As a result, vision will be blurred. This condition of defect of vision is called edema. Vision is blurred during corneal abrasion. The vision is blurred during corneal abrasion because the beam of light entering eye is either blocked or disoriented due to a scar or deformed shape of the cornea. FACTS WITH REASONS ACTIVITY 4 Make a model to represent corneal abrasion. Eye donation (cornea transplantation) Many people suffer from corneal injuries and have lost vision. They can regain their vision by corneal transplantation. The entire cornea or part of cornea is collected from the donor and stored. That graft is used to replace an entire cornea or part of our cornea with a minor surgery. Thus, the transfer of healthy cornea from donor to the receiver is called cornea transplantation. It helps to restore vision and reduce pain. Sometimes grafted cornea is rejected by the body. Figure to represent cornea transplantation Fig: corneal abrasion A person having a hypermetropic eye can see distant objects clearly but cannot see the nearby objects clearly. MEMORY TIPS The healthy transparent tissue of cornea is collected from a recently dead donor who does not contain any infectious, life threatening diseases. MEMORY TIPS

Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 269 Corneal transplantation removes blurred vision. The corneal transplantation removes blurred vision because the scarred or deformed cornea is replaced by a healthy and suitable cornea. So light beam can pass appropriately into the retina. FACTS WITH REASONS ACTIVITY 5 Make a pamphlet to raise public awareness of corneal transplantation. Colour blindness Our eyes have cone cells and rod cells. The cone cells help to see the colour images. If there are certain defects in the pigments on the cone cells or if some pigment is missing then the person cannot see particular colours. The inability to see certain colours in the usual way is called colour blindness. Red-green colour blindness is the most common colour blindness. Besides it, people can suffer from blue-yellow colour deficiency and fully colour blindness. Colour blindness can occur in any gender but it is common in men. ACTIVITY 6 Observe the picture given above and try to find out what is written in it. Example of an Ishihara colour test plate. The number "74" should be clearly visible to viewers with normal colour vision. Viewers with red-green colour blindness will read it as "21" and viewers with monochromacy may see nothing. Night blindness People with night blindness have no problem seeing during daylight or in well-lighted places. However, these people have difficulty seeing things in dim lights or at night time. The inability to see properly in poor light conditions is called night blindness. Malfunctioning rod cells due to lack of vitamin A is the primary cause of night blindness. Malfunctioning pupils and shortsightedness are also responsible for night blindness. Consumption of food rich in vitamin A can help to cure night blindness. We should eat sufficient green vegetables and yellow fruits. We should eat sufficient green vegetables and yellow fruits because they supply us with vitamin A which prevents night blindness. FACTS WITH REASONS Colour blindness is a genetic disorder. It is hereditary. MEMORY TIPS

270 lenS Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Figure of night blindness suffered eye Figure of source of vitamin A Eye cataract A disease in which the eye-lens becomes cloudy is called eye cataract. The main cause of eye cataracts is old age. However, it is also caused by air pollution, smoking or using alcohol heavily. It can be a defect by birth too. The build-up of protein clouds the eye’s lens. Cataracts will blur the vision and eventually cause blindness. The cloudy patches grow very slowly but continuously. If it is not treated on time, a person goes blind. Eye cataracts can be treated successfully by surgery. ACTIVITY 7 Collect some pictures of eyes with eye cataracts and make a collage to raise public awareness. Cataract surgery Cataract surgery is a process in which natural cloudy lens is replaced by intraocular lens. There are multiple ways for surgery. The most common cataract surgery is phacoemulsification. In this process a small cut is made at the side of an eye. A tip of machine vibrates at ultrasonic frequency and the lens will be broken into tiny pieces. The broken materials are absorbed by vacuum tube. Then an intraocular lens is installed in the eye. Intraocular lens is an artificially prepared lens from silicon. Figure of eye cataract Figure of eye cataract laser surgery Defects of Vision and Their Correction Objects 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 Collect some pictures of eyes with eye cataracts and make a collage to raise public awareness. MEMORY TIPS The loss of power of accommodation of an eye results in the defect of vision. MEMORY TIPS

Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 271 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-sightedness In 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 sightedness The 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, why? The eye lens of the myopic eye is thick with less focal length. Ciliary muscles are unable to pull the lens into thin shape. So, the refracted rays converge in front of the retina to make image. FACTS WITH REASONS ii) 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-sightedness To 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 lens Image Image Eye lens Affected by myopia Correction of myopia Dr. 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 TIPS Sign 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

272 lenS Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur b) Hypermetropia or long-sightedness Hypermetropia is the defect of vision in which a person can see distant objects clearly but cannot see nearby objects. 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-sightedness The 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, why? An eye suffering from long sightedness has longer focal length. Ciliary muscles are unable to compress the lens into thick shape. So, in case of an eye suffering from hypermetropia the refracted rays converge behind the retina. FACTS WITH REASONS ii) 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-sightedness To correct a hypermetropia, the person has to use spectacles with a convex lens of suitable focal length. The light rays from a nearby object are first converged by the convex lens and then by eye lens to form final image on the retina. Convex lens Image Eye lens Eye lens Image Affected by hypermetropia Correction of hypermetropia Contact lenses The contact lenses are transparent curved and soft lenses that are put on the eyes directly. It is usually used to correct visual defects. However, some people use it for attractive appearance of the eyes or to change the colour of the eye. Various types of contact lenses are available. It is a better alternative to spectacles. It also 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 Contact lenses are so safe that only 1 out of 500 people get a serious eye infection. MEMORY TIPS

Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 273 improves peripheral vision. Therefore, it is popular among athletes. It is used to correct shortsightedness, long-sightedness and other visual defects. Figure of contact lenses Figure of using contact lenses on eyes Contact lenses are replacing spectacles. Contact lenses are replacing spectacles because they are easy to use, do not get in the way during sports, do not fog up and provide better vision than spectacles. FACTS WITH REASONS ACTIVITY 7 Make a picture of contact lenses on A4-sized paper and enlist its advantages. Laser eye surgery [ Laser-assisted in situ Keratomileusis (LASIK)] People may suffer from various defects of vision such as myopia, hypermetropia, astigmatism and many others. Wearing spectacles or contact lenses are popular methods. However, laser eye surgery is a suitable alternative. Laser eye surgery is a method to correct defect of vision. It reshapes the cornea itself so that light will refract and reach the cornea properly. During LASIK surgery, a special type of cutting laser called excimer is used to precisely change the shape of imperfectly shaped cornea to improve vision. Laser eye surgery can treat corneal blindness. Laser eye surgery can treat corneal blindness because it changes the shape of cornea in such a way that the refracted light beam reaches the retina to form a clear image. FACTS WITH REASONS ACTIVITY 8 Many people may be afraid of laser eye surgery. How would you convince them that it is not harmful technology? Laser surgery of eye can be completed within a couple of minutes. MEMORY TIPS Figure of performing laser eye surgery

274 lenS Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur HOT SKILL HIGHER ORDER THINKING SKILL 1. A biconvex lens can be used to burn the paper. A biconvex lens can be used to burn the paper. It is possible because a biconcave lens is a converging lens. It can converge parallel beams of light passing through it at the principle focus. When light is converged at a single point on paper, it can increase the temperature of the paper. When the temperature is raised considerably the paper will start to burn. 2. Mariya Pun is a taekwondo player. She prefers contact lenses instead of glasses. Why does she prefer it? Mariya Pun is taekwondo player. She prefers contact lenses over glasses because of the following reasons: i. It can be used while playing games. It won’t fall. ii. It will not be covered by fog or water droplets or dust. iii. It increases peripheral vision. iv. It is convenient and easy to use. 3. Rijan studies in grade six. 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? As informed above, Rijan does not eat vegetables or fruits. He had difficulty seeing in the dim light in the morning and late at night. He usually bumps into furniture also. The reason is night blindness. He might have suffered from night blindness because he is not eating enough vegetables and fruits. His body is lacking vitamin A. He should do the following things to recover from night blindness. i. Consume green vegetables, milk and eggs. ii. Consume yellow fruits, pumpkins and carrots. iii. Consume vitamin A. iv. Using corrective lenses or glasses can help to improve vision. 4. Salina says the power of her lens is – 2.5 D. Which type of lens is she using? What kind of defect of vision does she suffer from? Calculate the focal length of that lens. Salina says the power of her lens is – 2.5 D. It means she is using a concave lens. Since she is wearing a concave lens, she must be suffering from short-sightedness. Solution: Power of the lens (P) = - 2.5 D Focal length (f) =? Using formula, f = 1 P = 1 2.5 = 0.4 m = 40 centimetres 5. Image distance for an eye is always constant. Why is it constant? How is it kept constant? Discuss. Image distance is always constant despite varying object distance so that the final image would always be formed at the retina. The image distance is kept constant by changing the focal length of the lens. The focal length of the lens is changed by changing the shape of the lens. The shape of the lens is changed by the contraction and relaxation of ciliary muscles and suspensory ligaments.

Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 275 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. When 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 lens Image Image Eye lens Affected by myopia Correction of myopia 7. 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 car 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.

276 lenS Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur 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. Convex lenses can converge parallel beams of light to make images. Under what condition it can make a virtual magnified image? Mention the possible uses of the ability of the convex lens to make such images. Convex lenses can converge parallel beams of light to make images. They usually make a real image. However, If an object is kept between focus and the optical centre, a convex lens can make a virtual and magnified image. The ability of the convex lens to make virtual and magnified images can be used: i. to make hand lenses to observe tiny parts of plants and flowers. ii. to make magnifying glasses to read small letters. iii. by watchmakers to magnify small parts of the watch iv. to make simple microscopes for science labs. 3 STEPS 3 STEPS EXERCISE EXERCISE STEP1 1. Select the best answer from the given alternatives. a. Why can the 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 b. An image formed in the plane mirror is not a real image. Why? i. reflected light rays do not actually meet. ii. reflected rays of light meet to form an image. iii. it is laterally inverted. iv. it is the same size as the object c. Why is an eye lens flexible? i. it is a crystal. ii. to see coloured images iii. to help us see objects at varying distances iv. to control the amount of light that enters the eye d. 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 e. What is the remedy for long-sightedness? i. vitamin A supplements ii concave lens iii. convex lens iv. green vegetables

Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 277 2. Define the following terms with required examples. a. Light b. Convex lens c. Centre of curvature d. Radius of curvature e. Principle axis f. Focal length g. Image h. Virtual image i. One dioptre j. Laser surgery k. Night blindness l. Contact lens m. Myopia n. Focusing o. Concave lens p. Aperture q. Optical centre r. Principle focus s. Colour blindness t. Eye cataract u. Accommodation of eye v. Hypermetropia w. Power of the lens 3. Answer the following questions in very short. a. What happens to the ray of light passing through the optical centre of a lens? b. Write the SI unit of the following: i. focal length ii. power of a lens c. What are the general features of the image formed by a diverging lens and converging lens? d. If an object is placed at the focus of a convex lens, where is the image formed? e. Where should an object be placed in front of a convex lens to obtain a magnified and erect image of it? f. An object is placed between the optical centre and the focus of a convex lens. What are its features? g. Write down the magnification formula for a lens in terms of object distance and image distance. h. Where does the human eye form the image of an object? i. What is the value of the least distance of distinct vision for a normal eye? j. What is the range of vision for a normal healthy human eye? k. Which muscles help the human eye to attain the power of accommodation? l. What is done in laser surgery? m. What happens in corneal injury? STEP2 4. Give reasons. a. Convex lens has a real focus. b. Power of a convex lens is positive. c. Concave lens has a virtual focus.

278 lenS Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur d. Power of a concave lens is negative. e. Convex lens is called a converging lens. f. It is dangerous to look at the sun through a convex lens. g. Concave lens is called a diverging lens. h. The power of a lens is measured as the reciprocal of its focal length. i. The refracted rays converge in front of the retina in case of an eye suffering from myopia. j. The refracted rays converge behind the retina in case of an eye suffering from hypermetropia. k. Concave lens is used for correction of short-sightedness. l. Convex lens is used for correction of long-sightedness. m. Children should eat enough green vegetables to avoid night blindness. n. Defects in cone cells can cause colour blindness. 5. Differentiate between the following. a. Real image and virtual image b. Concave lens and convex lens c. Far point and near point d. Myopia and hypermetropia e. Spectacles and contact lens 6. Answer the following questions in short. a. Write two uses of the concave lens and convex lens. b. What is focusing? How is it done? c. Define the following parts of an eye and write their functions. i. Cornea ii. Iris iii. Eye lens iv. Ciliary muscle v. Retina vi. Optic nerve d. A man can read the number plate of a distant bus clearly, but he finds difficulty in reading a book. Name the type of defect he is suffering from. Write two causes of such a defect. How can it be corrected? e. A child in a classroom is not able to read clearly the writing on the blackboard while sitting in the last row. Name the type of defect he/she is suffering from. Write two causes of such a defect. How can it be corrected? f. Which lens can correct short sight and how? g. Which lens can correct long sight and how? h. Where should we place an object in front of a convex lens so that the image can be as big as the object? Represent it in a ray diagram. i. 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? j. What is an intraocular lens? What are its benefits?

Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 279 k. Write a short note on laser surgery for the treatment of defects of vision. l. If an object is between focus and optical centre of the lens and the image is virtual, erect and diminished, mention the type of lens. What would be the type of lens if the image was virtual, erect and magnified? STEP3 7. Answer the following long questions. a. Complete the given ray diagrams in your exercise book and write down the position and nature of the image formed. b. Observe the given figures and answer the following questions: i) Which type of defect of vision is shown in the given figure? ii) Write two causes of the defect. iii) Which type of lens is used to correct the defect? c. Draw the ray diagram if an object is placed at different positions in front of a convex lens: i. At infinity ii. Beyond 2F iii. At 2F iv. At F v. Between F and 2F vi. Between F and O d. A person can see distant sign boards clearly but cannot read clearly on a book which is 25 cm away from his eye. Identify the defect. Draw a labelled ray diagram to show this defect and illustrate with a diagram how this defect can be corrected. e. Draw ray diagrams to show the formation of the image of an object by a concave lens when the object is placed between infinity and the optical centre of the lens. f. Observe the given figure and answer the following questions. ` 2F F' C F 2F A B i. Copy and complete the given ray diagram. ii. Write down the nature of the image thus formed.

280 lenS Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur iii. If the focal length of this lens is 20cm, calculate its power. iv. Write down the application of the image formed. g. Spectacle of Appy is + 0.5D and that of Loyala is + 1D. Who is wearing a thicker lens? How can you tell? 8. Numerical a. A concave lens has a focal length of 20cm. Calculate its power. [Ans: -5D] b. A person having a myopic eye uses a concave lens of focal length 10 cm. What is the power of the lens? [Ans: -10D] c. An object of height 5cm is placed at a distance of 40cm in front of a convex lens of focal length 20cm. Find the size of the image formed. Also, find the magnification of the lens and its power. [Ans: (I) = 5cm, (m) = 1, (P) = 5D ] d. An object of 5cm height is placed at 2F of a convex lens. Where is its image formed? What is the size of the image formed? Find its magnification. [Ans: 2F, 5cm, 1] e. A convex lens has a focal length of 3cm. A candle is placed at a distance of 4cm 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] f. A convex lens of power 10D 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] g. How far a hand lens of power 20D should be placed from the book to read it clearly and comfortably? [Ans: 5cm] h. If a person wears spectacles with lenses of power – 4.5 D: i. What is the type of corrective lens? ii. What kind of defect of vision does he have? iii. Find the focal length of the lens used. [Ans: 22.22 cm] i. A person wears spectacles with lenses of power + 3D: i. What is the type of corrective lens? ii. What kind of defect of vision does he have? iii. Find the focal length of the lens used. [Ans: 33.33 cm]

Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 281 Key terms and terminologies of the unit 1. Current: The rate of flow of electrons in a definite direction is called electric current. 2. Circuit: A circuit is a continuous conducting path to flow electrons from source of electricity through wires and appliances (electric loads) and back to the source. 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): If the current always flows in the same direction in a conductor, it is called direct current (DC). 6. Alternating current (AC): The current whose direction changes periodically is called an alternating current (AC). 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 UNIT Electricity and Magnetism 11 Estimated teaching periods Theory Practical 9 3 Michael Faraday shares the title of father of electricity with Nikola Tesla and Thomas Edison for his contribution to electrolysis, diamagnetism and the discovery of electromagnetic induction. He was born in England on 22nd September 1791 and died on 25th August 1867. He was an excellent experimental physicist. He was honoured with the Royal medal, Copley medal, Rumford medal and Albert medal. Michael Faraday About the Scientist

282 electRicity and magnetiSm Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur 7. Fluorescent lamp: A discharge lamp coated with fluorescent material from inside is called fluorescent lamp. 8. Magnetic effect: When current is passed through a wire, it produces a magnetic field around it. This effect of an electric current is called magnetic effect. 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: Electric motor is a device which converts electrical energy into mechanical energy on the basis of motor effect. 16. Transformer: A transformer is a device that converts high voltage AC to low voltage AC and vice versa. 17. Step-up transformer: The transformer which converts low voltage AC to high voltage AC is called step-up transformer. 18. Step-down transformer: The transformer which converts high voltage AC to low voltage AC is called step-down transformer. Introduction Electricity 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. Current Electromotive 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.

Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 283 Electric Circuit Electricity flows through a path called a circuit. A circuit is a continuous conducting path to flow electrons from the source of electricity through wires and appliances (electric loads) and back to the source. The basic components of an electric circuit are source, conducing wires, electric load, switch, etc. There are two types of electric circuits. They are: i. Open Electric Circuit An open circuit does not have a complete path to flow current. Thus, the electric circuit through which current cannot flow due to an interrupted path is called open electric circuit. Circuit becomes open when the switch is turned off or the wire is broken. Electric load does not work in an open circuit. ii. Closed Electric Circuit A closed circuit has a complete path to flow current. Thus, the electric circuit through which current can flow in an uninterrupted path is called closed electric circuit. Circuit becomes closed when the switch is turned on and the wire is uninterrupted. Electric load works in a closed circuit. Direct Current (DC) and Alternating Current (AC) Direct Current Electric 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 Current In 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 alternatively 50 times in one second. Switch Cell Copper wire Bulb Electric circuit Switch off Bulb Source Open circuit Source Switch (on) Bulb Closed circuit Current time graph of DC Time Current Current time graph of AC Current Time Rectifier converts AC to DC to run mobile, computer, etc. MEMORY TIPS Current time graph The graphical representation of the change in current in a conductor against change in time is called current-time graph. MEMORY TIPS

284 electRicity and magnetiSm Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Average voltage and frequency used in the mainline of Nepal Each country supplies predefined voltage, frequency and plug type for domestic and industrial use. However, a large region can use common standards. Many countries including the United States use 110 to 120 volts and 60-hertz frequency. Many other countries including Nepal use 230 volts and 50-hertz frequency. FACTS WITH REASONS The 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. Differences between AC and DC. S.N AC S.N. DC 1. Alternating current (AC) changes magnitude and polarity. 1. Direct current (DC) does not change magnitude and polarity. 2. It is produced by AC generator, dynamo, etc. 2. It is produced by cells, battery, DC generator, etc. 3. AC voltage can be changed by using a transformer. 3. DC voltage cannot be changed by using a transformer. 4. It has non-zero frequency. 4. It has zero frequency. Magnetic Effect of Current: Relation between Electricity and Magnetism The 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 wire A 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.

Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 285 ACTIVITY 1 1. 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. Magnetic field around a solenoid A solenoid is a cylindrical coil of wire. When electric current is passed through a solenoid, it behaves like a magnet. ACTIVITY 2 1. 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. One end of the current-carrying solenoid acts like a north-pole (N-pole) and the other end a south pole (S-pole). If a current carrying solenoid is suspended freely, it will rest in the north and south direction. Factors affecting the strength of a solenoid i. Total number of turns in the solenoid: By increasing the number of turns in the solenoid, the strength of an electromagnet is increased. ii. Current in the coil: If the current in the solenoid is increased, the strength of the solenoid increases. iii. Nature of core material: By using soft iron in the core of the solenoid, the strength of the electromagnet is increased. Solenoid is a temporary magnet. So, it is used to make electromagnet.

286 electRicity and magnetiSm Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Direction of the Magnetic Field The direction of magnetic field is from south to north pole of a magnet within the magnet and the field lines will go from north to south outside of the magnet. Maxwell's Right hand grip rule An electric current passes through a solenoid, resulting in a magnetic field. When you wrap your right hand around the solenoid with your fingers in the direction of the conventional current, your thumb points in the direction of the magnetic north pole. Magnetic Flux Magnetic flux is a method for determining the total amount of magnetic field in a particular area. It is a useful tool for explaining how the magnetic force affects something in a certain area. Thus, the number of magnetic lines of force passing through a certain area of a conductor held perpendicularly is called magnetic flux. If magnetic lines are close together, the magnetic field is strong. Similarly, if magnetic lines are far apart, the magnetic field is weak. At the magnetic poles, where the magnetic field is strongest, the lines are closer together. 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. motion conducting wire current magnetic lines S of force n Motor effect Some of the useful aspects of motor effects are listed below: i. Electric motors are made on the principle of motor effect. They are used in various sectors like in washing machine, water pump, rice mill, blenders, grinders, electric fans etc. ii. Electric cars, hair dryers and CD players also work on motor effects. Fig: Maxwell's Right hand grip rule Fig: Magnetic flux Motor effects is impossible without magnets. MEMORY TIPS

Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 287 iii. Construction tools such as electric drills, electric saws and air conditioner has electric motors. Fan electric drill blender electric flour mill FACTS WITH REASONS Kinetic energy is produced on an electric motor when we pass electricity in the conductor which is placed inside a magnetic field. Kinetic energy is produced on an electric motor when we pass electricity in the conductor which is placed inside a magnetic field because the magnetic field produced on the conductor is deflected by the permanent magnetic field of the motor. ACTIVITY 3 Find small electric motors or buy a new one and use it to make a model of an electric fan. ACTIVITY 4 1. Take a conductor and a conducting wire. 2. Make a support of the conducting wire at two points 'A' and 'B'. 3. Hang the conductor on a stand as shown in the given figure and connect the two terminals of the conductor with a battery. 4. Bring a U- shaped magnet close to the conductor. Do you observe a movement of the conductor? The motion of the conductor is due to the motor effect. Electric Motor An electric motor is a device which transforms electrical energy into mechanical energy. It works on the principal of motor effect. Thus, electric motor is a device which converts electrical energy into mechanical energy on the basis of motor effect. In an electric motor, electricity passes through a coil of wire producing a magnetic field around it. This coil is surrounded by permanent magnets and made free to spin. 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 Electric motor

288 electRicity and magnetiSm Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur 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 motor In a motor, the turning force on its coil depends on: i. the strength of the magnetic field ii. the amount of the current supplied iii. the number of turns in the coil Applications of electric motor Electric 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. Electromagnetic Induction Moving 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 lines of force. 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. Fleming's right hand rule Fleming'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 TIPS ACTIVITY 5 1. 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. Galvanometer Turns of wire Match box Magnet S Electric motor Electromagnetic induction

Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 289 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. Electromagnetic induction was discovered in 1831 by Michael Faraday. 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 TIPS In 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 Induction Depending on above experiments, Michael Faraday has given the laws of electromagnetic induction in 1831 AD which are as follows: 1. Whenever magnetic flux linking a coil or a conductor changes, an emf is induced in it. 2. The magnitude of induced emf is directly proportional to the rate of change of magnetic flux. i.e. induced emf ∝ change in magnetic flux time taken 3. The induced emf in the coil circuit lasts as long as the change of magnetic flux linking the coil continues. FACTS WITH REASONS A stationary magnet does not induce a current in a coil, why? In case of a stationary magnet, the magnetic lines of force are not intersected by the closed circuit or a coil. So, there is no change in magnetic flux. As a result, stationary magnet does not induce a current in a coil. Generator or Dynamo A generator, or dynamo, is a device which converts mechanical energy into electrical energy. It works on the principle of electromagnetic induction. A generator produces current on a large scale, whereas a dynamo does it on a small scale. Bicycle Dynamo A bicycle dynamo is a very simple dynamo in which a magnet is made to spin round near a fixed coil. Electromagnetic induction is the principle behind the generation of electricity in a bicycle dynamo. It converts mechanical energy into electrical energy. A dynamo produces alternating current (AC) whereas a dry cell produces direct current (DC). MEMORY TIPS

290 electRicity and magnetiSm Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Construction A bicycle dynamo has a cylindrical permanent magnet with poles on opposite sides of the cylinder. Its coil is fixed on the iron core. Working mechanism When the cylindrical magnet rotates, the magnetic lines of force are intersected by the coil and an emf gets induced due to change in magnetic flux. It results in induced current and lights the bulb. The current is induced in the bicycle dynamo as long as there is motion in the tyre. FACTS WITH REASONS The brightness of the bicycle dynamo bulb increases when the speed of the bicycle increases, why? When the speed of the bicycle increases, the number of magnetic lines of force intersected by the coil in the bicycle dynamo increases. It increases the rate of change of magnetic flux which increases the induced current and brightness of the bulb. ACTIVITY 6 Bring a bicycle dynamo and connect the rotating end of the dynamo to the tyre of a bicycle. Rotate the tyre at variable speed. Observe the brightness of the bulb by increasing and decreasing the speed. What do you observe? Conclusion: The brightness of the bulb increases on increasing the speed of the tyre and brightness of the bulb decreases on decreasing the speed of the tyre. Generator An electric generator is a device that converts mechanical energy into electric energy on the basis of electromagnetic induction. It is used to generate electricity on a large scale. It has a strong permanent magnet. When the turbine is rotated by an external force, the rotor fixed to the magnet will also rotate. As a result, current is induced in the coil due to the change in the magnetic flux linked to the coil. There are two types of generators, i.e. a.c. generator and d.c. generators. Structure of AC Generator Powerful electromagnets are present in the AC generator to generate powerful magnetic fields around the coil. The conducting coil inside the magnetic field is called armature. It is made of soft iron core to increase the rate of production of current. It has two ends. There are two slip rings in AC generator each at one end of the armature. Whenever the armature rotates current will be induced in it. When armature rotates half turn current is collected by first slip ring. When armature rotates another half turn, current changes direction and it is collected by second slip ring. So, the direction of induced current keeps changing. Bicycle dynamo Magnetic poles AC voltage Slip rings Carbon brushes Working of AC generator

Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 291 The carbon brushes are in contact with slip rings. They collect electricity from slip rings. Total number of complete rotations made by armature per second is called frequency. AC generators are used in hydro power stations, wind farms and thermal power stations. Each country supplies predefined voltage, frequency and plug type for domestic and industrial use. However, a large region can use common standards. Many countries including the United States use 110 to 120 volts and 60-hertz frequency. Many other countries including Nepal use 220 volts to 240 volts and 50-hertz frequency. Frequency of AC can be controlled by controlling the rotation of the turbine. If the turbine of generator rotates 3000 times per minutes (50 times per second), it can produce current of frequency of 50Hz. Voltage is directly proportional to frequency. If rate of rotation of armature is increased, frequency and voltage will increase. Methods to increase the induced emf from dynamo and generator i. We can increase induced current of the generator by increasing the number of turns in the coil. ii. We can increase induced current of the generator by increasing the speed of the rotation. iii. We can increase induced current of the generator by decreasing the distance between the magnet and the coil. iv. We can increase induced current of the generator by increasing the strength of the magnetic field by using a powerful magnet. Differences between an electric motor and generator. Electric motor Generator 1. An electric motor converts electrical energy into mechanical energy. 1. A generator converts mechanical energy into electrical energy. 2. It is based on the motor effect. 2. It is based on the principle of electromagnetic induction. 3. Current is supplied to the coil placed in a magnetic field by an external source. 3. The coil is rotated in the magnetic field by an external source. Big Sources of Electricity 1. Hydro-power Hydro-power is the energy that comes from the force of moving water. It can be found in those areas where water flows at high speed from high ground to low ground. To generate hydroelectricity, water is collected in a dam. The collected water is passed through a tunnel. The force of water can be used to rotate turbines When current is passed through a generator, or dynamo, it becomes an electric motor. So, an electrical generator can be run "backwards" to form a motor. MEMORY TIPS technology of hydroelectricity

292 electRicity and magnetiSm Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur connected with the generator which produces electricity. Such electricity generated from the fast flowing water is called hydroelectricity. Advantages of hydroelectricity i. Hydroelectricity does not cause air pollution. ii. It can be transmitted over long distances in extremely small interval of time. iii. It can be converted into different forms of energy like heat, light, etc. by passing through suitable devices. iv. Water used to generate hydroelectricity can be reused for other purposes like irrigation. v. It is cheaper in long term use though the installation cost of a hydro-power plant is very high. 2. Wind Energy The energy obtained from the moving wind is called wind energy. This energy is a result of unequal heating of the earth surface and the atmosphere. It is one of the oldest and cleanest forms of energy. A large amount of energy can be produced from the windmill. It depends upon weather and location. Wind energy is used for grinding grain, pumping water, or generating electricity. The kinetic energy of wind rotates the turbines connected with the generators which generate electricity. Wind power plants or wind farms are a collection of many wind turbines to generate electricity. Applications of wind energy i. Wind energy is used to propel sailboats. ii. Wind energy rotates the turbines connected with generator to generate electricity. iii. It is used to pump water from well. iv. It is used to grind wheat and other grains in grinding mills. Advantages i. Wind is a renewable source of energy, we never run out of it. ii. It is a clean source of energy. It does not cause pollution. iii. It is free of the cost and is reliable. iv. Wind farms can be built off-shore. 3. Thermal energy The energy produced from the temperature of the heated substance is called thermal energy. A thermal power station can use thermal energy to produce electricity. The main concept of thermal power stations is to use heat energy to convert water into steam and use the pressurized steam to rotate a steam turbine which will eventually produce electricity. Any source of heat can be used to convert water into pressurized steam. For example, fossil fuel, nuclear energy, geothermal energy, biofuel, solar energy, waste incineration, etc. The design of Wind farm

Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 293 a thermal power plant varies based on the source of heat. The thermal power plant is bad for the environment because it emits lots of greenhouse gases into the environment. Figure of thermal power plant Working mechanism A coal-powered thermal power station burns coal to generate electricity. The coal is pulverized and burned in a pulverized coal-fired boiler. The heat converts water into very hot steam. The pressurized fast flowing steam is passed into the turbine. The turbine rotates to produce electricity. Advantages of thermal energy i. It is a cheaper source of electricity. ii. Coal is cheap and easily available to produce thermal energy. iii. It can use geothermal energy, solar energy or waste incineration also to produce thermal energy. iv. There is less cost to make a thermal power plant. FACTS WITH REASONS Thermal power plants should be discouraged. Thermal power plants should be discouraged because it is one of the major sources of air pollution, global warming and climate change. ACTIVITY 7 Make a 3D model of a thermal power plant. Worldwide there are about 8,500 coal-fired power stations totaling over 2,000 gigawatts. MEMORY TIPS

294 electRicity and magnetiSm Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Transformer The transmission of electricity over a long distance with the same potential difference is very difficult. Potential difference decreases when current is transferred for a long distance. Similarly, high voltage current cannot be used for the domestic purposes. At the hydro-power station or nuclear power plants, the electricity is first generated at a lower potential difference. It must be stepped up to a high voltage. Similarly, when electricity enters a building or home, the high voltage alternating current (AC) should be stepped down to a low voltage alternating current (AC). This increase and decrease of the voltage can be done with the help of a device called transformer. Thus, a transformer is a device that converts high voltage AC to low voltage AC and vice versa. Construction A transformer has two coils fixed in a soft-iron core. The main parts of a transformer are, a) Primary Coil : It is the coil connected to the source of alternating current. The number of windings in the primary coil are called primary turns (n1 ) and the AC voltage supplied in primary coil is called primary voltage (V1 ). b) Secondary Coil : It is the coil through which the stepped-up or stepped-down AC voltage is obtained. The number of windings in the secondary coil are called secondary turns (n2 ) and the AC voltage obtained from secondary coil is called secondary voltage (V2 ). c) Soft iron laminated core : A block made from U-shaped iron strips or E and I shaped iron strips to fix the primary coil and secondary coil of the transformer is called core. The process of joining polished U-shaped iron stripes together is called lamination of the core. For insulation of iron strips shellac or varnish is used to polish them. FACTS WITH REASONS The core of a transformer is laminated, why? During lamination of the core of a transformer, the soft iron strips are separated from one another by the layers of insulator. Due to lamination of the core, production of eddy current in the core is reduced. It prevents the core from excessive heating as well as energy loss. So, the core of a transformer is laminated. Working Principle Alternating current passes through the primary coil when it is connected to an AC source. The magnetic field due to AC in primary coil changes in both magnitude and direction. This changing magnetic field changes the magnetic lines of force linked with the secondary coil. Transformer A.C. input A.C. output Iron core Secondary Coil Primary Coil Magnetic field in core Construction of transformer

Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur Modern Concept Science & Technology - 10 295 According to the Faraday's laws of electromagnetic induction, whenever the magnetic lines of force linked with a closed circuit changes, an emf gets induced from two terminals of the secondary coil. Thus, a transformer is based on the principle of mutual induction. The principle of mutual induction states that "an emf gets induced in a coil by passing an alternating current in the neighbouring coil." FACTS WITH REASONS A transformer does not work with DC, why? Instead of AC, if DC is supplied in the primary coil, then the magnetic field produced is of constant polarity. Due to this, the number of magnetic lines of force which are intersected by the secondary coil does not change. In such condition, current does not get induced from the two terminals of the secondary coil. So, transformer does not work with DC. A transformer cannot change the emf of a dry cell,why? A transformer cannot change the emf of a dry cell because dry cell produces only DC current. The magnitude and polarity of the DC does not change. So, a transformer cannot change the emf of a dry cell. A transformer is based on following two laws i. According to the law of conservation of energy, the input power at the primary coil (P1 = I1 V1 ) is equal to the output power at the secondary coil (P2 = I2 V2 ). ii. The magnitude of induced e.m.f. is directly proportional to the ratio of the number of turns in the secondary coil (n2 ) to the number of turns in the primary coil (n1 ). Transformer Formula The relation among primary voltage (V1 ), primary turns (n1 ), secondary voltage (V2 ), and secondary turns (n2 ) in a transformer is called transformer formula. It is given as: Secondary voltage (V2 ) Primary voltage (V1 ) = Secondary turns (n2 ) Primary turns (n1 ) or, V2 V1 = n2 n1 Types of Transformer There are two types of transformers. They are step-up transformer and step-down transformer. a. Step-up transformer The transformer which converts low voltage AC to high voltage AC is called step-up transformer. It has more turns in secondary coil than in primary coil, i.e. n2 > n1 . So, it has more secondary voltage than primary voltage, i.e. V2 > V1 . Applications i. It is used to get high voltage to accelerate the electron beam in a cathode ray tube (CRT). ii. At the hydro-power station or nuclear power plants, the low voltage AC is stepped up Output Input AC source 220V 11000V Step up transformer

296 electRicity and magnetiSm Approved by the Curriculum Development Centre, Sanothimi, Bhaktapur to a high voltage using a step-up transformer. iii. In every microwave oven step-up transformer changes input voltage. iv. Inverter transformer converts low voltage from the battery into high voltage AC. v. It is also used in television sets, refrigerator, etc. b. Step-down transformer The transformer which converts high voltage AC to low voltage AC is called step-down transformer. It has more turns in primary coil than in secondary coil, i.e. n1 > n2 . So, it has more primary voltage than secondary voltage, i.e. V1 > V2 . Applications i. In a distribution sub-station, a step-down transformer is used to lower the high voltage AC to low voltage AC for domestic use. ii. Battery chargers like cell phone charger have a step down transformer. It reduces high voltage (from 220 V) to low voltage (between 3 V to 9V) iii. It is also used in radios, cassette players, etc. Differences between Step-up transformer and Step-down transformer S.N. Step-up transformer S.N. Step-down transformer 1. A step-up transformer converts low voltage AC to high voltage AC. 1. A step-down transformer converts high voltage AC to low voltage AC. 2. The number of turns in the secondary coil is more than that in the primary coil. 2. The number of turns in the secondary coil is less than that in the primary coil. 3. In this transformer, secondary voltage is greater than primary voltage. 3. In this transformer, primary voltage is greater than secondary voltage. Solved Numerical 11.1 The power supply to the primary coil of a transformer is 220 V and the turns in the primary coil are 2000. What must be the number of turns in the secondary coil to run a radio of 11 V from this transformer? Solution: Given, Primary voltage (V1 ) = 220 V Primary turns ( n1 ) = 2000 Secondary voltage (V2 ) = 11 V Secondary turns ( n2 ) = ? According to the transformer formula, V2 V1 = n2 n1 or, 11 220 = n2 2000 AC source Input 11000V Output 220V Step down transformer