Oasis School Science and Technology 10 New

Oasis School Science and Technology - 10 251 The constant is called refractive index and is denoted by µ where µ is a Greek letter and pronounced as 'mew'. The second law of refraction of light gives the relationship between the angle of incidence and the angle of refraction. This relation was discovered by Willebrord Snell in 1621 AD. Therefore, this law is also called Snell's law of refraction. This law explains the refraction in the following conditions. Rules for the Bending of Rays at the Interface of Two Optical Media i. When a ray of light travels from an optically denser medium to a rarer medium, it bends away from the normal and when it passes from a rarer medium to a denser medium, it bends towards the normal. Reasonable Fact For a ray of light travelling from an optically rarer to an optically denser medium, the angle of incidence is always greater than the angle of refraction. A ray of light travelling from a rarer medium to a denser medium slows down and bends towards the normal at the point of incidence. So, for this ray of light, the angle of incidence is always greater than the angle of refraction. For a ray of light travelling from an optically denser to an optically rarer medium, the angle of incidence is always less than the angle of refraction. A ray of light travelling from a denser medium to a rarer medium goes faster and bends away from the normal at the point of incidence. So, for this ray of light, the angle of incidence is always less than the angle of refraction. ii. A ray of light passing through normal does not deviate. In this case, the angle of incidence is zero. Activity 1 • Place a coin at the bottom of an empty bowl. Move your eye away from the bowl till you can no longer see the coin. Now, ask your friend to pour some water into the bowl. Observe what will happen. The coin becomes visible to us. Why? Fact File If a ray of light travels from air to water along a certain path, it retraces the path, when light travels from water to air. The path of light is reversible. Through the normal (c) Towards normal (b) Away from normal (a) Denser (glass) Normal Normal r r i i Denser (glass) Denser (glass) Rarer (air) Rarer (air) Rarer air Fig.

252 Oasis School Science and Technology - 10 Refractive Index The ratio of the sine of angle of incidence to the sine of angle of refraction for a given pair of media is constant which is called refractive index. It is denoted by µ. Since the refractive index is a ratio of two similar quantities, the sine of angles, it has no unit. It is a pure number. If we consider a ray of light travelling from air to glass medium, then the ratio sin i to sin r is called the refractive index of glass with respect to air, i.e. sin i sin r = airµ glass The refractive index of several media is given in the table. Medium Absolute Refractive Index Vacuum 1 Ice 1.31 Water 1.33 Ethyl Alcohol 1.36 Kerosene 1.44 Glass 1.50 Crown Glass 1.52 Diamond 2.42 Worked out Numerical 1 Calculate the refractive index of a glass medium if the angle of incidence and angle of refraction are 45° and 28° respectively. Solution: Angle of incidence (i) = 450 Angle of refraction (r) = 280 Refractive index (µ) = ? We have, µ = sin i sin r = Sin 450 Sin 280 = 1.5 ∴ The refractive index of glass with respect to air is 1.5. Refractive index of a medium can also be defined as the ratio of speed of light in vacuum to the speed of light in the medium, i.e. Refractive index (µ) = sin i sin r Fact File In comparing two media, the one with the larger refractive index is an optically denser medium than the other. The other medium of lower refractive index is optically rarer. Refractive index (µ) = Speed of light in vacuum (c) Speed of light in medium (v)

Oasis School Science and Technology - 10 253 Worked out Numerical 2 Calculate the refractive index of water if the speed of light in vacuum and water are 3 × 108 m/s and 2.2 × 108 m/s respectively. Solution: Speed of light in vacuum (c) = 3 × 108 m/s Speed of light in water (v) = 2.2 × 108 m/s Refractive index (µ) = ? We have, µ = c v = 3 ×108 2.2×108 = 1.36 ∴ The refractive index of water is 1.36. Refraction of light through a glass slab Consider a rectangular glass slab PQRS having parallel faces PQ and RS as shown in the figure on the next page. A ray of light AB in air medium is incident on the glass surface PQ at the point B. Since the ray AB enters from air (rarer medium) to glass (denser medium), the ray bends towards the normal and follows the path BC. At point C, refraction takes place again. Since the ray BC enters from glass (denser medium) to air (rarer medium), the ray bends away from the normal, emerging out of the glass and following the path CD. The ray AB is called the incident ray, BC is called refracted ray and CD is called the emergent ray. longer path shorter path A N P Q lateral shift Glass slab R S i r e B N' M M' X D Y C Fig. Refraction of light through a glass slab Incident ray, AB Refracted ray BC Emergent ray, CD Lateral shift, XY Normal NN' and MM' Angle of incidence, ∠ABN Anlge of refraction, ∠N'BC Emergent angle, ∠M'CD In the above figure, it is seen that the emergent ray CD is parallel to the incident ray AB, but the emergent ray is laterally displaced from the original path of the incident ray by the perpendicular distance XY. The perpendicular distance between the original path of the incident ray and the emergent ray is called lateral displacement or lateral shift. Lateral displacement is directly proportional to the thickness of the slab, incident angle and refractive index of the glass.

254 Oasis School Science and Technology - 10 Activity 2 Draw two lines of 10 cm in length each, perpendicular to each other and intersecting at the midpoint. Put the protractor at the intersection as shown in the diagram and mark from 0°C to 90°C in the first quarter and third quarter. Now put a semicircular glass slab at the x-axis as shown in the diagram. Pass ray of light from third quarter using laser light at an incidence angle of 30°, 42° and 80° respectively and measure the corresponding angle refraction. Find the ratio of sin i to sin r in each case. What do you observe? Real Depth and Apparent Depth When the ray of light travelling in a denser medium enters the rarer medium, it bends away from the normal. Due to this phenomenon, the bottom of a pond appears to be raised and the depth seems to be less than that of the actual one. The actual depth of the pond is called real depth. But the virtual depth at which a body appears due to refraction of light is called apparent depth. Therefore, a coin lying at the bottom of a cup full of water appears to be raised. Similarly, the bottom of a swimming pool filled with water appears to be raised. In the given diagram, when we observe from point S, point O seems to be coming from I. So, AI is called apparent depth and AO is called real depth. The refractive index in terms of real depth and apparent depth is given by where µ is the refractive index of the denser medium with respect to the rarer medium. The refractive index of water with respect to air is 4 3 = 1.33. Therefore, the depth of a pond appears 3 4 th of its real depth while viewing it from vertically above. Worked out Numerical 3 Calculate the refractive index of water if the real depth and apparent depth of the bottom of a pond is 3 m and 2.25 m respectively. Solution: Real depth = 3 m Apparent depth = 2.2 m Refractive index (µ) = ? Fig. real depth and apparent depth Real depth Apparent depth Water Coin I O A A N N' i r Refractive index (µ) = Real depth Apparent depth Fact File Snell's law is not applicable when the angle of incidence is equal to zero, i.e., when the ray is incident along the normal.

Oasis School Science and Technology - 10 255 We have, µ = Real depth Apparent depth = 3 2.2 = 1.33 ∴ The refractive index of water is 1.33. Effects of Refraction of Light 1. When light travels from a denser medium to a rarer medium, it bends away from the normal. In our daily life we observe a large number of phenomena based on this principle. Some of them are as follows: i. Bending of stick in water A stick immersed in water appears to be bent and short at the surface of water when it is viewed obliquely from above. The rays of light coming from the lower end of the stick bend away from the normal as light passes from water (denser medium) to air (rarer medium). As a result, a virtual image of the part of the stick below water is formed as shown in the figure. Thus the immersed part of the stick appears to be raised and bent on the surface of water. ii. A coin placed at the bottom of a vessel full of water appears to be raised The rays of light coming from the coin bend away from the normal as light passes from water (denser medium) to air (rarer medium). As a result, a virtual image of the coin is formed above the real position due to refraction of light. Therefore, a coin placed at the bottom of a vessel full of water appears to be raised. Similarly, more phenomena can be explained on the basis of the principle of refraction of light, when it travels from a denser medium to a rarer medium. i. A water pond or a pool appears less deep than its actual depth. ii. The legs of a person standing in a swimming pool appear shorter. iii. During spear fishing, the fisherman aims at the tail of the fish. iv. A piece of paper lying at the bottom of the glass slab appears to be raised when viewed through the glass. v. The print appears to be raised up when a glass is placed over it. Reasonable Fact If a spear is shot exactly at the place where the fish is seen in water, the fish does not get killed, why? The fish inside water appears slightly higher than its actual position due to refraction of light. As a result, the fish does not get killed if a spear is shot exactly at the place where the fish is seen in water. Fig. Bending of stick in water Water Apparent bend Apparent position of stick Surface Fig. Water Coin (real position)

256 Oasis School Science and Technology - 10 2. When light travels from a rarer medium to a denser medium, it bends towards the normal. A number of phenomena can be observed in our daily life based on this principle. Some of them are as follows: i. The sun is visible two minutes before the actual sunrise and two minutes after the actual sunset The actual sunrise takes place when the sun is just above the horizon. When the sun is just below the horizon, the light rays coming from it suffer atmospheric refraction when they enter a denser medium (lower atmospheric layers) from a rarer medium (upper atmospheric layers). So they bend towards the normal at each refraction. Due to the continuous refraction of light rays coming from the sun at each layer of atmosphere, they follows a curved path as shown in the figure and reach the observer's eye. As a result, we can see the sun two minutes earlier than it rises above the horizon in the morning due to atmospheric refraction of light. It is also due to atmospheric refraction of light that we can still see the sun for two minutes even after the sun has set below the horizon. Thus a day would have been shorter by four minutes if the earth was without an atmosphere. Fig. ii. The stars seem higher than they actually are The stars seem to be higher in the sky than they actually are due to the atmospheric refraction of light. The light rays coming from the stars suffer atmospheric refraction on entering earth's atmosphere as light rays come from a rarer medium to a denser Fig. 5.8 Earth Observer Horizon Apparent sun Rising sun

Oasis School Science and Technology - 10 257 medium. So they bend towards normal at each refraction. Due to the continuous refraction of light rays at each layer of the atmosphere, they follow a curved path as shown in the figure. As a result, the stars seem higher than they actually are. Refractive index of air is increasing Apparent position Real position Fig. iii. Stars twinkle but planets do not twinkle The twinkling of stars is due to atmospheric refraction of light rays coming from the star. The rays of light coming from the star suffer atmospheric refraction due to varying optical densities of air at various altitudes. The continuously changing atmosphere refracts the light from stars by varying amounts and in different directions from one moment to the next. When the atmosphere refracts more starlight towards us, the star appears to be bright and when the atmosphere refracts less starlight, the star appears to be dim. In this way, the amount of starlight reaching our eyes increases and decreases continuously. As a result, stars twinkle at night. On the other hand, planets do not twinkle at night. The planets are very close to the earth as compared to the stars. So, the intensity of light that we receive from planets is very large. So small variations in their position and brightness are not noticeable. Thus, the brightness of a planet always remains the same. The continuously changing atmosphere is unable to cause variations in the light rays coming from big planets. Therefore, planets do not twinkle at night. Critical Angle A ray of light bends away from the normal when it passes from a denser medium to a rarer medium. Let's observe the given diagram. For ray b, the angle of incidence is less than the angle of refraction. When the angle of incidence in the denser medium is increased gradually, the corresponding angle of refraction also increases. For a certain angle of incidence in the denser medium, the a b b a 90° c d d c r Fig. Refraction of light and total internal reflection i

258 Oasis School Science and Technology - 10 corresponding angle of refraction becomes 900 . In the diagram, the ray c gives the angle of refraction 900 . This particular angle in the denser medium is called critical angle. Fig. Critical angle and total internal reflection c r i i i r r = 900 Rarer Rarer Rarer Denser Denser Denser The angle of incidence in the denser medium for which the corresponding angle of refraction in the rarer medium is 900 is called critical angle. It is denoted by C or ic . For example, the critical angle for ordinary glass is about 420 with respect to air. It means that when the angle of incidence of a ray of light in glass is 420 , the angle of refraction in air is 900 . Total Internal Reflection When the angle of incidence is further increased beyond the critical angle, the ray of light reflects back to the same denser medium instead of refraction. This phenomenon is called total internal reflection of light. During this process, no light is refracted or transmitted or absorbed by the surface of separation. The total incident light is reflected back into the denser medium. Conditions for total internal reflection i. The ray of light must pass from a denser medium to a rarer medium. ii. The angle of incidence in the denser medium must be greater than the critical angle. Critical angles for some substances with respect to air are given below: S.N. Substances Refractive Index Critical Angle 1. Ice 1.31 500 2. Water 1.33 490 3. Alcohol 1.36 480 4. Paraffin 1.44 440 5. Turpentine 1.47 430 6. Glycerine 1.47 430 7. Glass 1.5 420 8. Diamond 2.42 240 Relation between the refractive index of a medium and the critical angle When a ray of light passes from a denser medium (glass medium) to a rarer medium (air medium), then according to Snell's law, We have, glassµair = Sin i Sin r prism /ˈprɪzəm/ - a transparent glass or plastic object, in the shape of a triangle, which separates light that passes through it into the colours of rainbow

Oasis School Science and Technology - 10 259 For the incident ray moving from a denser medium to a rarer medium, the angle of refraction is 900 and the refracted ray is parallel to the glass-air interface. Thus, i = ic (critical angle) and r = 900 . Then, glass µ air = 1 sin ic Here,air is the refractive index of the denser medium (glass) with respect to the rarer medium (air). Thus, if the refractive index of the denser medium with respect to rarer medium is known, the critical angle (iC) can be calculated. Total Internal Reflection in a water Water is a transparent liquid in which light travels at the velocity of 225, 000 kilometres per second. The refractive index of water is 1.33 and the critical angle of the water-air interface is 48.6⁰. Therefore a ray of light travelling from water (denser medium) to air (rarer medium) and incident on the water-air interface at an angle greater than 48.6⁰, suffers total internal reflection. Air (n2 ) Water Refracted ray Total internal reflection Critical angle Incident ray 35° 48.6° 50° n n n q2 q1 c q1 q2 Fig: Total internal reflection in the water Due to the multiple total internal reflections of light, laser light can pass through a stream of water as shown in the diagram. glass air c c glass air air i i But µ µ µ = = =     = sin sin sin sin 90 90 1 1 0 0 Q glass c air glass glass air c i or i ∴ = =       sin sin 1 1 µ µ glass air c c glass air air i i µ µ µ = = =     = sin sin sin sin 90 90 1 1 0 0 Q glass c air glass glass air c i i = =    sin sin 1 1 µ µ

260 Oasis School Science and Technology - 10 Fig: Multiple total internal reflections in a stream of water Worked out Numerical 4 Calculate the critical angle of water medium with respect to air if the refractive index of water is 1.33. Solution: Re ( ) . ( ) ? , sin fractive index Critical angle i We have i o c c µ µ = = = 1 33 1 r i or i or i Sin c c c 1 33 1 1 1 33 1 1 33 1 . sin sin . . = = = ( ) − = 480 45' ≈ 490 The critical angle of water medium with respect to air is 490 . Total Internal Reflection in a Prism A prism is made of glass. Since the refractive index of glass is about 1.5, the critical angle of glass with respect to air is about 420 . Therefore, a ray of light travelling from glass (denser medium) to air (rarer medium) and incident on glass-air interface at an angle greater than 420 suffers total interval reflection. Fig. R Q P 450 450 450 450 450 Q R R 450 450 900 450 450 P Q 900 450 R 450 450 450 Inverted image Erect image

Oasis School Science and Technology - 10 261 A prism having an angle of 900 between two refracting surfaces and remaining angles being equal to 450 is called a total reflecting prism. Such prism is generally used, i) to deviate a ray of light through 900 . ii) to erect the inverted image without deviation. iii) to deviate a ray of light through 1800 . Application of Total Internal Reflection at Prism The ability of total internal reflection by a prism is used to make optical devices such as periscopes, binoculars, microscopes, single-lens reflex cameras, etc. The prism does not absorb or refract or reflect any rays of light. Every ray of light suffers total internal reflection and travels to another point. So it makes a clear image. Prism also helps to convert inverted images into erect ones so that it will be easy to use binoculars. Fig: periscope with prism Fig: binoculars with prisms Fig: binoculars with prisms Consequences of Total Internal Reflection 1. Sparkling of a cut diamond The critical angle for diamond in air medium is very low, i.e. about 240 . The faces of diamond are cut in such a way that once a ray of light enters into it entering angle of incidence is greater than the critical angle. So, a ray of light passing through diamond suffers repeated total internal reflection at a large number of faces. As a result, several faces of the diamond appear silvery. Therefore, a cut diamond sparkles. 2. Light pipe A transparent hollow tube made of glass or plastic is called light pipe. It is made of a bundle of highly transparent and flexible glass fibres called optical fibres. The light ray that falls on each optical fibre gets internally reflected as shown in the figure. Fig. Light pipe Uses: Light pipes are used by the doctors to view the internal organs of a human body. Fig.

262 Oasis School Science and Technology - 10 Such device is called endoscope. Light pipes are also used for telecommunication for transmitting signals. They are also used to transmit the images of the objects. Reasonable Fact Internal parts of human body can be seen with the help of a light pipe. Light pipe is made up of optical fibres. When light pipe is inserted inside the human body, the light rays coming from internal parts of body suffer total internal reflection. As a result, the internal parts of the body can be seen. 3. Mirage Mirage is an optical illusion of water in which a pond-like structure is seen on the surface of coal-tarred roads or in the hot desert. In this phenomenon, an inverted image of a distant object is seen along with the object itself on a hot sunny day. It is caused due to the total internal reflection of light in upward direction. In mirage, the inverted image of a distant object appears as if it were the reflection from the surface of water. Mirage is formed on a very hot day when the surface of the road or desert is heated and the air molecules above it become hot. It results in the formation of different layers of air of different density. The density increases from surface of the road. The air with high density acts as a denser medium. So the light travelling down from upper layer of air bends away from the normal as it goes down from cold layers to hot layers of air. After coming down through a number of layers, the ray of light suffers total internal reflection and it starts going in upward direction. As a result, we see the mirage. 4. Shining of air bubble in water The bubble of air rising up in an aquarium appears silvery due to total internal reflection of light. The critical angle for water-air interface is 490 . When the ray of light propagating in water (denser medium) is incident on the surface of the air bubble (rarer medium) at an angle greater than 490 , total internal reflection takes place. As a result, the bubble of air in water shines brightly. Shining of a Test Tube Under Water A test tube under the water shines brightly when viewed from a certain angle. It happens because of the total internal reflection of light. The air inside the test tube is a rarer medium and the water around it is a denser medium. So, the ray of light which goes toward an air bubble (rarer medium) from water (denser medium), at an angle of incidence greater Virtual image Fig. Mirage Rarer layer of air Denser layer of air Tree Air bubble i>c Fig. shining air bubble Fig: Shining of a test tube under water

Oasis School Science and Technology - 10 263 than the critical angle, will suffer total internal reflection. The air in the test tube acts like a convex mirror, reflects the light and shines brightly. Reasonable Fact Sound is not heard at a distance during the day. During the day land becomes warm and so does the air above it. The warmer air rises. As a result, a layer of air near the ground becomes the rarer medium and the upper layers become denser. When we speak, our sound, similar to light, has to pass from a warm rarer medium. So, it bends toward the normal. As a result, the sound waves will travel to higher altitudes and cannot be heard by the person on the ground a few meters away. Sound can be heard at a distance during the night. At night, the air near the land is colder and acts as a denser medium. The upper layers of the air act as a rarer medium. When we speak the sound has to travel from denser to rarer medium. If sound travels at an angle of incidence greater than the critical angle, it will bend away from the normal and reflect back to the ground. So, sound can be heard at a distance during the night. Application of Total Internal Reflection of Light Optical fibre Optical fibre is a very thin, transparent tube slightly thicker than human hair made from silicon or plastic. Optical fibres are based on the total internal reflection of light. Usually, an optical fibre consists of three parts. They are the core, cladding and jacket. Fig: structure of the optical fibre Core The innermost flexible transparent thin cylinder of glass is called the core. It is the path through which light can travel from one end to another by multiple total internal reflections of light. It has a higher refractive index and acts as a denser medium. Cladding It is the second layer of the optical fibre. It is coated around the core. It is a transparent medium made of glass. Its refractive index is lesser than that of the core. It acts as a rarer medium.

264 Oasis School Science and Technology - 10 Jacket The jacket is the outermost, flexible protective cover of the optical fibre. Working mechanism of optical fibre When light is passed from one end, it tries to pass through the denser core to a rarer cladding medium. If the angle of incidence is greater than the critical angle, the light suffers total internal reflection. The process repeats multiple times so that light can come out from another end. Optical fibres are widely used in medicine and telecommunication. A. Use of optical fibre in medicine Optical fibres are modified into laparoscope, endoscope and colonoscope. They help to diagnose diseases in the abdominal region, alimentary canal and large intestine respectively. 1. Endoscopy Endoscopy is the process in which an endoscope is used to examine the interior of the intestine, blood vessels, lungs etc. An endoscope is a modified light pipe. It is a flexible tube with a camera and a source of light at the insertion tip. An endoscope is based on the total internal reflection of light. Endoscopy is done to review the alimentary canal, uterus and respiratory tracts. It helps to detect ulcers, gastrointestinal disorders, mucosal disorders etc. 2. Colonoscopy Colonoscopy is the process in which an endoscope is passed through the anus into the large intestine to check the health condition. It helps to see the cause of bleeding, abdominal pain, rectal cancer etc. 3. Keyhole surgery Keyhole surgery is a surgical procedure in which the surgeon can access the abdomen and pelvis with the help of a laparoscope. A laparoscope is a modified light pipe. It contains a small and thin tube, a light source and a camera. It works on the principle of multiple reflections of light. It helps to perform surgeries without large cuts. A laparoscope, a small surgical tool and a tube are passed into the abdomen through a tiny hole at the abdomen up to the required place by controlling it with a controller. Fig: endoscopy Fig: colonoscopy Fig: keyhole surgery using a laparoscope Fact File The advantage of keyhole surgery is a shorter recovery time and lesser chance of infection.

Oasis School Science and Technology - 10 265 Reasonable Fact Patients recover faster after keyhole surgery. Patients recover faster after a keyhole surgery because smaller incisions cause less pain, less bleeding, and faster healing. Activity 3 Discuss the advantages of keyhole surgery. Make a chart and paste it into the classroom. B. Use of optical fibre in telecommunication Optical fibres are made up of flexible light pipes. They can transfer light from a source to longer distances. So optical fibres are used by many telecommunication companies to transport telephone signals, internet and television signals. The infrared light bounces off the optical fibre multiple times and transmits voice, video and telemetry through a local network or long distances. Internet Service providers use a router connector to connect an optical fibre to the router device. Then the router can connect our devices to the internet. Fig: optical fibres help in telecommunication Fig: fibre router connector Reasonable Fact Optical fibres can transfer data from source to receiver The optical data can transfer data because the internal surface of the fibre is reflective. The light suffers total internal reflection many times and reaches the destination. Activity 4 Observe the optical fibre if available. Fact File Data travels faster through fibre optic cables.

266 Oasis School Science and Technology - 10 Differences between Reflection and Total Internal Reflection S.N. Reflection S.N. Total Internal Reflection 1. A smooth surface like mirror is required for reflection of light. 1. An optically denser and an optically rarer media are required for total internal reflection. 2. It takes place at any angle of incidence. 2. It occurs only when the angle of incidence in denser medium is greater than its critical angle. 3. Some percentage of light is absorbed by the reflecting surface. 3. The entire incident light reflects back totally without absorption at the interface of two media. Dispersion of Light When a beam of light is passed through a prism, it splits into seven different colours. This process is called dispersion of light. The band of seven colours obtained on the white screen is called spectrum. The seven colours of the spectrum are: violet (V), indigo (I), blue (B), green (G), yellow (Y), orange (O) and red (R). It shows that the white light is a mixture of seven colours. So, the phenomenon of splitting white light into its constituent colours on passing through a glass prism is called dispersion of light. This phenomenon was discovered by Newton by passing sunlight through a prism in his dark room rainbow is formed due to dispersion of light. When the sunlight passes through the rain water, it gets dispersed in seven colours. As a result rainbow is formed. Cause of Dispersion of Light Light travels in the form of wave. It has a very short wavelength (average wavelength is 5 × 10-4 mm). Among seven colours, red ray has the longest wavelength (i.e. 6.2 × 10-7 m to 7.8x10-7) whereas violet ray has the shortest wave length (i.e. 3.8 × 10-7 m to 4.4 × 10-7). The wavelength of indigo is 4.4 × 10-7 m to 4.6 × 10-7 m. The blue has wavelength of 4.6×10-7 m to 5.0x10-7 m. The green has wavelength of 5.0 × 10-7 m to 5.8 × 10-7 m. Wavelength of yellow ranges from 5.8×10-7 m to 5.9x10-7 m. The wavelength of orange is 5.9×10-7 m to 6.2 × 10-7 m. The wavelength increases gradually from violet to red. It is to be noted that all the rays have the same speed in the vacuum. But they have different speed in a denser medium. The ray having the longest wavelength has the highest speed whereas the rays with shorter wavelength have slower speed. So, the violet ray has the lowest speed and red ray has the greatest speed. Therefore, the dispersion of white light occurs because the angle of refraction or angle of bending of different colours is different when passing through the glass prism. While passing out from the opposite face of the prism, they do not suffer any dispersion but suffer refraction. As a result, they separate out further and form a band of colours on a white screen which is called the spectrum. Fig. Dispersion of light through a glass prism White light beam Glass prism VIBGYOR

Oasis School Science and Technology - 10 267 Activity 5 • Take a right angled prism. Incident a light ray normally on its one face in a dark room. Observe how the ray travels. (a) (b) Fig. When the light ray strikes the second face of the right angle prism, the angle will be 45°, which is more than 42°(critical angle of glass). Therefore, the ray suffers total internal reflection. Due to the reflection of all the rays of light there is no change of the intensity. This is the reason why prisms are used in binoculars and periscope instead of mirrors. The light shows the dispersion when it passes through the prism but not through the glass zslab, why? A prism has a triangular shape. When a ray of light strikes a surface it bends towards the base of the prism and gets dispersed. Furthermore they refract in the same direction when they emerge out from another surface of the prism. White spot Coloured light Beam of white light Fig. (a) (b) A D E C B F White light beam Glass prism White light spectrum A glass slab can be considered as the combination of two prisms as shown in the fig.(b). When one prism ABC splits the rays of light, another prism DEF combines the seven colours of spectrum to give back white light on emerging out of the second prism. In the same manner, the dispersed rays of light recombine by forming white light while coming out of the parallel face of the glass slab. So, there is no dispersion of light through the glass slab. Newton's disc The circular disc which has seven segments with seven colours of rainbow in each segment is called Newton's colour disc. When the disc is rotated the colours disappear. This experiment where seven colours merge into one when disc is rotated is called newtons disc experiment. It is also called disappearing colour disc. It is used to demonstrate that a white ray of light is made of combination of seven colours of rainbow. Fig: Newton's disc

268 Oasis School Science and Technology - 10 Rainbow A rainbow is a semicircular or circular colourful meteorological phenomenon seen in the sky. It is usually formed after a light rainfall. It is observed in the opposite side of the sun. The rainbow is formed when a ray of light enters the water droplet. First of all light refracts while entering from air to water. It reaches the inside of the droplet and disperses into seven colours. The seven colours will suffer total internal reflection and refract again while leaving the water droplet. If the seven colours suffers total internal reflection twice double rainbow will be formed. Reasonable Thinking Skill Reasonable Thinking Skill R T S 1. The 6 feet deep swimming pool does not appear as its depth. Even though the pools are deep or they do not seem that way. If a pool is actually 6 feet deep, it looks like four feet. The reason for this is the refraction of light. To see an object, light reflected by it must reach our eye. When the light reflected by the bottom of the pond has to pass from water into the air, it bends away from the normal and reaches our eye. But our eye will perceive reflected light as a straight line. So, the image of the bottom of the pond is formed slightly above the actual position. 2. When I was looking at scenes from the window of the bus, I noticed that objects were slightly raised when seen from the glass window than when seen from the open window. How does it happen? When observed from a glass the scenes from the bus window were slightly raised than seen from the open window. The reason is the refraction of light. The glass is a denser medium. When the light reflected from the surroundings passes into the glass, it bends toward the normal. Then it emerges from the glass into the air. It bends away from the normal. It will be bent twice during the process. As a result, the light will be laterally shifted by a certain length. As a result, it appears that the surrounding is slightly lifted when observed through the glass. 3. Ramesh has had gastric problems for many years. He has acute pain in the abdomen. He was admitted to the hospital. The doctor inserted a tube through his mouth and they were able to locate an ulcer in his intestine. What was the device used, on what principle does it work and how does it work? As informed, we learnt that a long tube was inserted through his mouth to observe any injury in the intestine. The device is an endoscope. It is a modified light pipe. It works on the principle of total internal reflection of the light. The working mechanism of the endoscope: It has a long tube that can be directed and moved with the help of the computer. It has Fig: formation of rainbow

Oasis School Science and Technology - 10 269 a tiny camera that can take videos and photos and show them on the computer. It is gently inserted down the throat. Doctors control it remotely and navigate the internal parts of the body. 4. After a shower, rainbows are seen in the sky. Give reason. After a shower rainbows are seen in the sky. When there is shower there are lots of tiny air drops in the air. When the sunlight passes through it from a certain angle, it will disperse into a colourful spectrum. Hence the rainbow is formed. 5. Mirage is formed in the desert during the daylight. Give reason. During hot sunny days, the sand becomes very hot and heats the air above it. The hot air rises. It leaves thinner air near the ground compared to the upper layers. So, when the ray of light passes from a denser medium to a rarer medium, it will undergo total internal reflection. Hence the sky is reflected in it and a mirage is formed. 6. A glass jewel cut in the same shape as a diamond does not shine much. Give reason. A glass jewel cut in the same shape as a diamond does not shine much because the critical angle of glass is 42° whereas that of a diamond is 24° only. So, there is too less total internal reflection of light in glass compared to the diamond. As a result, the glass jewel does not shine much even though it has the same shape as a diamond jewel. 7. Sanoni is a 8-year-old child. She is trying to hit a fish with coins but failed many times even if she shoots coins where the fish is seen. What could be the reason? What physical phenomenon cause this? Sanoni is a 8-year-old child who is trying to hit a fish with coins but failed every time. The reason behind it is, the fish is seen above its real position. The physical phenomenon responsible for this is the refraction of light. For a fish to be seen, the reflected rays of light from fish must pass from a denser medium, water, into the 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 Sanoni to pinpoint the location of the fish and fails to hit. 8. I was told that the sun we see during the sunset has actually, already set, a few minutes earlier. How was it already set a few minutes ago, if I can see it right now? Indeed, the sun we see during the sunset has actually set, a few minutes earlier. Even though the sun is set, we are still able to see it because of atmospheric refraction of light. The earth is surrounded by the atmosphere. When the sun sets behind the horizon, its parallel beam of light has to pass through the atmosphere. The upper layers are rarer and the lower layers are denser. So based on the laws of refraction, when a beam of light passes through a rarer to the denser medium they bend toward the normal. So, the sunlight bends downward and reaches us even when the sun has already set. 9. 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? 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

270 Oasis School Science and Technology - 10 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 will suffer 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 the copper wire because they can transfer light rays by multiple total internal reflections faster without any loss. Exercises 1. Choose the best answer from the given alternatives. a. Why does light bend when passing from air into the glass? i. because of the increase in velocity ii. because of the decrease in the velocity iii. because light travels in a straight line iv. because light refracts b. Why does a fish appear above its real position in water? i. light bends away from the normal when passing from water to air medium ii. light bends toward the normal when passing from air to the water medium iii. light normal to the medium does not bend at all iv. critical angle makes the fish appear up c. Under what situation a sea diver under the water, looking up toward the sky cannot see the sky but sees the part of the ocean below? i. if the critical angle is smaller than the angle of incident. ii. if the critical angle is equal to the angle of incidence iii. if the critical angle is greater than the angle of incidence iv. if the angle of refraction is 90 °. d. What is the principle in which light pipe works? i. reflection of light ii. refraction of light iii. total internal reflection of light iv. dispersion of light e. 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 f. What is responsible for the formation of a rainbow? i. reflection of light ii. refraction of light iii. diffraction of light iv. dispersion of light 2. Define the following terms with required examples. a. Refraction of the light b. Optically denser medium c. Optically rarer medium d. Refractive index e. Snell’s law f. Total internal reflection of the light g. Critical angle h. Light pipe i. Mirage j. Dispersion of the light

Oasis School Science and Technology - 10 271 3. Answer the following questions in very short. a. What is optical fibre? b. What is the velocity of light in air and water medium? c. What is an endoscope? d. What is the apparent image of water called which is seen in a desert or pitched road? e. How does light bend when it passes from a rarer medium to a denser medium? f. Which angle is equal to the angle of emergence during refraction of light through a glass slab? 4. Give reasons. a. Light bends when it passes from one medium to another. b. The depth of a pond appears less than the real depth. c. A coin placed in a beaker appears to be raised when the beaker is filled with water. d. If a spear is jabbed exactly at that place where the fish appears in the water, the fish is not killed. e. Diamond sparkles even in dim light. f. Mirage is seen on the coal-tarred road on hot days. g. Light pipe is used to see internal organs to diagnose diseases. h. There is no dispersion of light through the glass slab. i. Stars twinkle when observed from the earth but not from the moon. j. Air is a rarer medium when light passes through the air into the water. k. Palm of our hand seems bigger when put into the water. k. Glass cut in a similar shape to a diamond does not sparkle as much as a diamond. m. Sunlight disperses into multiple colours when it passes through the prism. 5. Differentiate between the following. a. Denser medium and rarer medium b. Refraction of light and total internal reflection c. Real depth and apparent depth d. Refraction of light and dispersion of light e. Refraction and reflection 6. Answer the following questions in short. a. What are the laws of refraction of light? Enlist. b. What are the conditions necessary for total internal reflection of light? Write any two applications of total internal reflection of light. c. What is an endoscope? Why is it used? d. What is the relationship between refractive index, real depth and apparent depth? e. In which condition is the mirage seen? Explain with a figure. f. Draw a neat diagram showing the dispersion of light through a glass prism. g. What causes light to disperse? h. Semi-circular rainbow can be seen after light rainfall on sunny days. How are these rainbows formed? i. Complete the given ray diagrams.

272 Oasis School Science and Technology - 10 Air (a) (b) (c) (d) Air Glass 90° 45° 42° 45° 7. Answer the following questions on the basis of the given figure. i. Name the rays A and B. ii. Which ray has more speed (A or B) in the glass medium? iii. Which phenomenon is shown in the given diagram? iv. Ray B is bending more than ray A. What is the reason behind it? v. How can we merge dispersed rays into white light? 8. Answer the following long questions. a. A ray of light is passing through another medium by making an angle of 45°. If the refractive index of the medium is 1.4, calculate the magnitude of the angle of refraction. The apparent depth of water in a pond appears less than the real depth. Why? b. The speed of light in air is 3 × 105 km/s and the refractive index of paraffin is 1.44. Calculate the speed of light in the paraffin. Stars twinkle but planets do not. Justify this statement. c. Describe the refraction of light through the glass slab with the help of a neat and clean diagram. d. What is the application of total internal reflection of light in modern human civilization? e. A light that passes through the triangular plastic scale forms a spectrum of colours. How can we explain this? 9. Answer the following questions on the basis of the given figure. i. Which phenomenon is shown in the given diagram? ii. Name the angle of incidence, angle of refraction and emergent angle in the given diagram. iii. Which medium is denser out of air and glass? Explain the reason. Light Prism A B shorter path A N P Q lateral shift Glass slab R S i r e B N' M M' X D Y C

Oasis School Science and Technology - 10 273 Key terms and terminologies 1. Lens : A lens is a piece of transparent glass bound by two refracting surfaces, which are usually spherical. 2. Convex lens : A lens which is thick in the middle and thin at the edges is called a convex lens. 3. Concave lens : A lens which is thin in the middle but thick at the edges is called a concave lens. 4. Focusing : The process of adjusting the distance between a lens and the screen so that the image becomes clear and distinct is called focusing. UNIT 10.2 LENS Estimated teaching periods Theory 6 Practical 2 • 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) The Sequence of Curriculum Issued by CDC 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. About the Scientist Euclid

274 Oasis School Science and Technology - 10 5. Real image : The image which is formed when light rays meet and that can be obtained on the screen is called a real image. 6. Virtual image : An image which is formed without meeting of light rays and cannot be obtained on the screen is called a virtual image. 7. Power of a lens : The ability of a lens to converge or diverge the rays of light falling on it is called the power of a lens. 8. One dioptre (1 D) : The power of a lens is 1D if its focal length is 1 metre. 9. 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. 10. 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. 11. 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 accommodation. 12. Colour blindness : The inability to see certain colours in the usual way is called colour blindness. 13. Night blindness : The inability to see properly in poor light conditions is called night blindness. 14. Cataract : A disease in which the lens becomes cloudy is called eye cataract. 15. Corneal injury : The damage to the cornea due to physical causes, chemical causes or diseases can affect the vision because they can leave scars or discolourations. 16. Defect of vision : The disorder in which a person cannot see nearby or distant objects clearly is called defect of vision. 17. Myopia : Myopia is the defect of vision in which a person can see nearby objects clearly but cannot see distant objects. 18. Hypermetropia : Hypermetropia is the defect of vision in which a person can see distant objects clearly but cannot see nearby objects. 19. Contact lenses : The contact lenses are transparent curved lenses that are put on the eyes directly. 20. Laser eye surgery : 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. Introduction A watchmaker uses a hand lens to view extremely small parts of a watch clearly. Spectacles are used to make things visible. In fact, a lens plays an important role in our every day life. A mirror works on the basis of reflection of light whereas a lens works on the basis of refraction of light. A lens is a piece of transparent glass bounded by two refracting surfaces, which are usually spherical. Lenses can refract parallel beam of light and either converge or diverge them in the process. Lenses are made from at least one curved transparent surface. Fig. Convex lens Convex surface Convex surface

Oasis School Science and Technology - 10 275 Types of Lenses On the basis of nature, there are two types of lenses. They are: i. Convex lens and ii. Concave lens i. Convex lens: A lens which is thick in the middle and thin at the edges is called a convex lens. A convex lens is also known as converging lens because it converges the parallel rays of light at a point after refraction. Since the rays of light actually pass through the focus of the convex lens, it has a real focus. Fig. 5.2 Converging action of a convex lens Fact File Convex lenses are of three types . They are: (i) Biconvex lens (ii) Planoconvex lens (iii) Concavoconvex lens Biconvex Planocovex Concavoconvex Reasonable Fact Convex lens is called a converging lens. A convex lens converges the parallel rays of light at a point after refraction. So, it is called a converging lens. Activity 1 Take a convex lens (hand lens) and a piece of paper. Focus the sunlight on the paper with a fine beam. What happens to the paper after 3 – 4 minutes? The paper starts burning after some time since the convex lens converges the parallel beams of light at a point after refraction. Repeat the activity with a concave lens. What happens? The paper does not burn since the concave lens diverges the parallel beams of light. Fig. Burning a piece of paper by using a hand lens Fact File The speed of light is very high. It travels at the speed of 300000km in one second through a vacuum.

276 Oasis School Science and Technology - 10 Reasonable Fact A convex lens has a real focus. 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. 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. ii. Concave lens: A lens which is thin in the middle but thick at the edges is called a concave lens. A concave lens is also known as diverging lens because it diverges the parallel rays of light after refraction. Since the rays of light do not actually pass through the focus of a concave lens, it has a virtual focus. Fig. Concave lens Fig. Diverging action of a concave lens Concave Concave (Focus) F (Focal length) (Focus) Principal axis Parallel rays Concave lens or diverging lens f F O Lens as a Group of Prisms A convex lens is made of a set of prisms in such a way that the prisms in the upper half have their bases downwards and the prisms in the lower half have their bases upwards. The central part of the lens acts as a glass slab. The prism in the upper part of the lens bends the incident ray downwards, while the prism in the lower part of the lens bends the incident ray upwards. The central part of the lens, which is just like a rectangular glass Fact File Concave lenses are of three types. They are: (i) Biconcave lens (ii) Planoconcave lens (iii) Convexoconcave lens Biconcave Planoconcave Convexoconcave

Oasis School Science and Technology - 10 277 block, allows the incident ray to pass undeviated. Similarly, a concave lens is made of a set of prisms in such a way that the prisms in the upper half have their bases upwards and the prisms in the lower half have their bases downwards. The central part of the lens is just like a glass slab. The prism in the upper part of the lens bends the incident ray upwards, while the prism in the lower part of the lens bends the incident ray downwards. The central part passes the incident ray undeviated. Terminology Related to Lens i. Centre of curvature (2F or C): The centre of the sphere from which the lens has been cut is called the centre of curvature (2F or C). A lens has two such points. Center of curvature Center of curvature 2F F F O 2F X Y X Y Principal axis 2F F F 2F Fig. Center of curvature Center of curvature O A A B B Concave lens Convex lens R R R R ii. Aperture: The aperture of a lens is the maximum portion of the spherical surface through which refraction takes place. In the above figure, AB is the aperture of the lens. iii. Optical centre (O): The geometrical centre of a lens is known as its optical centre. In the given figure, the point 'O' is the optical centre. A ray of light passing through it doesn’t suffer any deviation and goes straight ahead. iv. Principal axis: The line joining both the centres of curvature of a lens is called the principal axis. In the given figure, XY is the principal axis. v. Principal focus or focus (F): Principal focus is the point on the principal axis where the rays of light parallel to the principal axis converge after refraction through a convex lens or appears to diverge from a point on the axis through a concave lens. In the figure, the point 'F' is the focus. vi. Focal length (f): The distance between the principal focus and the optical centre of a lens is called the focal length. vii. Radius of curvature (2f): The radius of the sphere from which the lens has been cut is called the radius of curvature. It is the distance between the optical centre and centre of curvature. It is denoted by 'R' or 2f. Convex lens Fig. Concave lens

278 Oasis School Science and Technology - 10 (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 Fig. Reasonable Fact A concave lens has a virtual focus. The refracted rays from a concave lens appear to diverge from a point. When we produce them backward they meet at a point which is the virtual focus. So, a concave lens has a virtual focus. Process of Finding Focal Length of a Convex Lens When the object is at infinity, the distance of the image from the lens will be equal to the focal length of the lens. This is used to find the focal length of a convex lens. Take a convex lens whose focal length is to be determined. Keep the lens in front of a distant object and put a card board as a screen behind the lens. Change the distance of the screen from the convex lens until a clear inverted image of the distant object is formed on the screen. Measure the distance between the screen and the lens. The distance will be the rough focal length of that lens. Activity 2 Take a convex lens. Try to get an image on the screen. Measure the distance between the screen and the lens. This is the rough value of the focal length of the lens. Focusing: The process of adjusting the distance between a lens and the screen so that the image becomes clear and distinct is called focusing. It is done to produce a sharp and clear image in a camera, microscope, telescope, etc. Rules for Drawing Ray Diagrams Rule 1. The ray of light which is parallel to the principal axis of a convex lens always passes through the focus after refraction. In the case of a concave lens, the ray appears to be diverging from the principal focus.

Oasis School Science and Technology - 10 279 F O F Fig. O Convex lens Concave lens Rule 2. An incident ray passing through the principal focus of a convex lens becomes parallel to the principal axis after refraction. In the case of a concave lens, the incident ray appearing to meet the focus is refracted parallel to the principal axis. F O F Fig. O Convex lens Concave lens Rule 3. The ray of light that passes through the optical centre of a convex lens or a concave lens goes straight without deviation after refraction. O Fig. O Convex lens Concave lens Image and Its Types When an object is placed in front of a lens, the rays of light from the object fall on it and get refracted. The refracted rays produce a picture of the object, which is called the image. An image is a physical likeness or representation of an object produced by a lens after refraction. Lenses form two types of images. They are: i) Real image: The image which can be obtained on the screen is called a real image. It is always inverted. A convex lens generally forms a real image. This type of image is formed when the refracted rays actually meet at a point. ii) Virtual image: An image which cannot be obtained on the screen is called a virtual image. It is always erect. A concave lens always Fact File Convex lenses are very similar to concave mirror in their image forming properties. Fact File A convex lens can form both real and virtual images depending upon the distance between object and lens.

280 Oasis School Science and Technology - 10 forms a virtual image. This type of image is formed when the refracted rays appear to meet at a point. Images Formed by a Convex Lens 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 object can be placed: (i) at infinity (ii) beyond 2 F (iii) at 2 F or C (iv) between F and 2F (v) at F (vi) between F and O A brief description of the images formed by a convex lens is given below: 1. When an object is at infinity Fig. F B' D A' Image (Real, inverted) O F' B A Parallel rays from top point of a distant object Characteristics of the Image Position : At F Size : Highly diminished Nature : Real and inverted Utility : This type of image formation represents the action of the objective lens of an astronomical telescope and a camera lens focused at infinity. 2. When object is beyond 2F Fig. 2f Image (Real, inverted) Convex lens B 2F' F' A O F A' B' 2F Characteristics of the Image Position : Between F and 2 F Size : Diminished Nature : Real and inverted Utility : This type of image formation is used in a photographic camera. 3. When object is at 2F Fig. 5.13 Characteristics of the Image Position : 2 F Size : Same size as the object Nature : Real and inverted Utility : This type of image formation is used in a terrestrial telescope. 2f F O F 2F A B B' A' 2F Image (Real, inverted) Object Fig.

Oasis School Science and Technology - 10 281 A4. When object is between 2F and F Fig. Characteristics of the Image Position : Beyond 2 F Size : Enlarged (magnified) Nature : Real and inverted Utility : This type of image formation is used in film and slide projectors to produce an enlarged image on the screen. F' C F 2F A' B' 2F Object Image (Real, inverted and magnified) A D B 5. When object is at F X Characteristics of the Image Position : At infinity Size : Highly enlarged (magnified) Nature : Real and inverted Utility : This type of image formation is used in making search lights and spot lights in Fig. theaters. Image at infinity Rays become parallel Object Convex lens F A O F Y B f 6. When object is between F and O Fig. Image (virtual, erect and enlarged) Convex lens O F B A' F A B' Image Characteristics of the Image Position : Beyond the object Size : Enlarged (magnified) Nature : Virtual and erect Utility : This type of image formation is used in a simple microscope or a hand lens. Object Summary of the images formed by a convex lens Position of the object Position of image Size of image Nature of image 1. At infinity At F (focus) 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 (enlarged) Real and inverted 5. At F At infinity Highly magnified Real and inverted 6. Between F and O On the same side of the object (beyond object) Magnified Virtual and erect

282 Oasis School Science and Technology - 10 Image Formed by a Concave Lens When an object is placed anywhere between the optical centre and infinity, the image formed by a concave lens lies in between the principal focus and optical centre on the same side of the lens. The image is always diminished, virtual and erect. 1. When the object is kept anywhere between infinity and the optical centre Characteristics of the Image Position : Between F and O Size : Diminished Nature : Virtual and erect Utility : This type of image formation is used in spectacles for correcting myopia or short sightedness. Concave lens X Eye Center line of the lens (Virtual, erect and diminished) Object Image F B' A' B C Y A Fig. 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 light rays falling on it whereas a concave lens diverges. The ability of a lens to converge or diverge the rays of light falling on it is called the power of a lens. The power of a lens is defined as the reciprocal of its focal length in meters. Thus, Power of a lens (P) = 1 Focallength f( ) ∴ = P f 1 The SI unit of power of a lens is dioptre (D). The power of a lens is inversely proportional to its focal length. Thus, a lens of short focal length has more power and vice-versa. One dioptre (1 D): The power of a lens is 1D if its focal length is 1 metre. Physical Quantity SI unit Power D (dioptre) Focal length m (meter) ∴ = P f 1 Remember

Oasis School Science and Technology - 10 283 Fact File The instrument used to measure the power of a lens is called the dioptremeter. It is used by an optician to find the power of the lens of spectacles. Fact File A concave lens has negative focal length so it has negative power. A convex lens has positive focal length so it has positive power. Worked out Numerical 1 Calculate the power of a lens having focal length 10 cm. Also, write down the type of the lens. Solution: Here, Focal length (f) = 10 cm = 10 100 1 10 m m = Power of the lens (P) = 1 1 1 10 10 f = = D Since the focal length is positive, the lens is a convex lens. Reasonable Fact The power of a lens is measured as reciprocal of its focal length. A thick lens with short focal length has more ability to converge or diverge the light rays. But the thin lens with longer focal length has less ability to converge 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. Worked out Numerical 2 The power of a spectacle lens is –2.5 D. Calculate the focal length of the lens. Also, name the type of the lens. Solution: Here, Power (P) = –2.5 D Focal length (f) = ? We have, P = 1 1 1 10 10 f = = D or, – 2.5 = 1 1 1 10 10 f = = D ∴ f = 1 −2 5. = – 0.4 m Since the focal length is negative, it is a concave lens. 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

284 Oasis School Science and Technology - 10 of the 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. 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, e.g., a camera, telescope, microscope, binoculars, etc. Most of the optical instruments work on the basis of refraction and reflection of light. In this unit, we will discuss the structure and working mechanism of the human eye and defects of vision with ray diagrams. Human eye A human eye is also an optical instrument. The eye ball is more or less spherical in shape having a diameter of about 2.2 cm. Iris Sclerotic Retina Choroid Optic nerve Suspensory ligament Ciliary muscles Cornea Aqueous humour Fig: Structure of human eye i. Sclera or sclerotic layer : Sclerotic is the outermost covering of the eye. It is also called the 'white of the eye'. It protects the eyeball. ii. Cornea : The transparent front part of the eye is called the cornea. It is made of a transparent substance and is bulging outwards. The light coming from objects passes through the cornea. 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.

Oasis School Science and Technology - 10 285 iv. Iris: The coloured, ring shaped and flat membrane behind the cornea is called iris. It controls the size of the pupil. v. Pupil : Pupil is an adjustable circular opening at the centre of iris. It appears black since no light is reflected from it. vi. Eye-lens : The eye-lens is a convex lens which is made of proteins. The lens is supported and adjusted by ciliary muscles. vii. Ciliary muscle : The smooth muscle that helps in the accommodation of eye is called ciliary muscle. The focusing is done by the ciliary muscles. In this respect, it is different from a photographic camera. viii. Retina : The innermost, photosensitive part of the eye in which image is formed is called retina. The image is formed on the retina of the eye. The retina is made up of two types of photosensitive cells called rods and cones. ix Aqueous humour and vitreous humour :There are two types of liquid in the human eye; one is present in between the cornea and lens called aqueous humour and another in between the lens and retina called vitreous humour. These liquids help the eye ball to retain its shape. x. Optic nerve : The image formed on the retina activates the light sensitive cells, which generate electrical impulses. The retina sends those electrical impulses to the brain through the optic nerve. Although the image formed on the retina is inverted, the brain intercepts the erect and correct image of the object. For sometime, we cannot see objects clearly as we enter a dark room, why? When we enter a dark room from a bright light, at first we cannot see things clearly. After some time, our vision improves. This is due to the fact that in a bright light the pupil of our eye becomes small. So, when we enter a dark room, very little light enters our eyes, and we cannot see things clearly. After sometime, the pupil of our eye expands and becomes larger. More light enters our eye, and we can see things clearly. When we go into a bright light from darkness the case will be just the opposite. Accommodation A normal eye is one which does not have any defect of vision. Thus, a normal eye can see distant objects as well as nearby objects clearly. An eye can form the image on the retina for objects at various distances by changing the focal length of the eye lens. The changing of the focal length of the eye lens is done by the ciliary muscles. When we see distant objects, the ciliary muscles are relaxed and the eye lens becomes very thin. It helps to increase the focal length. As a result, the converging power of the eye lens decreases. Fig. Working mechanism of a human eye Lens Pupil Object Image

286 Oasis School Science and Technology - 10 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 Fig. In this position eye lens has large focal length but small converging power When we see nearby objects, ciliary muscles become tense due to which the eye lens becomes thick and focal length decreases. It increases the converging power of the eye lens. Here eye lens has short focal length but large converging power Image Ciliary muscles are tense Diverging rays from nearby object 25 cm O Fig. In this position eye lens has short focal length but large converging power 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 accommodation. Reasonable Fact We cannot see our surroundings clearly when we enter a dark cinema hall from a bright sunshine, but our vision improves after some time. The size of the pupil of our eye is small in the bright sunshine. So, when we enter the dark cinema hall, very little light enters our eyes through the small pupil, and we cannot see properly. After some time, the pupil of our eye expands and becomes larger to allow more light to pass through it, and we can see clearly.

Oasis School Science and Technology - 10 287 Reasonable Fact An object placed at a distance of less than 25cm cannot be seen by the normal human eye. The maximum accommodation of a normal human eye is reached when an object is located about 25cm from the eye. After this, the ciliary muscles cannot make the eye lens bulge more. Therefore, an object placed at a distance of less than 25cm cannot be seen clearly by the normal eye because all the power accommodation of the eye has already been used. Far point: The distant point from the eye which can be seen clearly is called the far point of the eye. For a normal eye, the far point is at infinity. Near point: The nearest point up to which an eye can see the object clearly is called the near point of the eye. For a normal eye, the near point is at a distance of 25 cm from the eye. Fact File Least distance of distinct vision: The minimum distance at which an object is clearly visible for a normal eye is called the least distance of distinct vision. For a normal eye, it is about 25 cm. Fact File Range of vision: The distance between the far point and the near point is called the range of vision. For a normal human eye, the range of vision is about 25 cm to infinity. Some Problems Related to Eyes 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. Colour blindness can be hereditary. Mutations and harmful radiations can also cause colour blindness. Red-green colour blindness (deuteranopia) 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 3 Observe the given picture 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. Fact File Colour blindness is a genetic disorder. It is hereditary.

288 Oasis School Science and Technology - 10 Night Blindness (Nyctalopia) 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 short-sightedness are also responsible for night blindness. Consumption of food rich in vitamin A can help to cure night blindness. Reasonable Fact 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. Eye cataract A disease in which the 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 cataract surgery. Normal Eye with contract Fig: eye cataract 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.

Oasis School Science and Technology - 10 289 Fig: eye cataract surgery Activity 4 Collect some pictures of eyes with eye cataracts and make a collage to raise public awareness. 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 can affect the vision 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. 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. Reasonable Fact 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. Fact File Dr. Sanduk Ruit invented a very cheap and high quality intraocular lens in 1995. The lenses are being used to treat eye cataract in Asia and Africa. Fig: corneal abrasion Fact File 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.

290 Oasis School Science and Technology - 10 Activity 5 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. Nepal eye bank harvest, collect, store and supply cornea if required. That graft is used to replace an entire cornea or part of our cornea with minor surgery. It helps to restore vision and reduce pain. Sometimes grafted cornea is rejected by the body. Reasonable Fact 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. Activity 6 Make a pamphlet to raise public awareness of corneal transplantation. Defects of Vision The disorder in which a person cannot see nearby or distant objects clearly is called defect of vision. There are two most common defects of vision which are: 1. Myopia or short sightedness 2. Hypermetropia or long sightedness 1. Myopia Myopia is the defect of vision in which a person can see nearby objects clearly but cannot see distant objects. It is also called short sightedness or near sightedness. For such an eye, the far point is less than infinity. Causes of myopia i. Due to the shortening of the focal length of the eye lens or high converging power of the eye lens ii. Due to the elongation of the eye balls Fig. cornea transplantation Fact File The healthy transparent tissue of cornea is collected from a recently dead donor who does not contain any infectious, life-threatening diseases.

Oasis School Science and Technology - 10 291 In this case, the ciliary muscles attached to the eye lens do not relax sufficiently to make the eye lens thinner to reduce the converging power. So, the image of a distant object is formed in front of the retina due to the great converging power of the eye lens. As a result, the person cannot see distant objects clearly. Fig (a) An eye affected by myopia Fig (b) Correction of myopia Concave lens Image Image Eye lens Remedy: Myopia can be corrected by using a concave lens of suitable focal length. When a concave lens of suitable power is placed in front of the eye having myopia, the parallel rays of light coming from the distant object are first diverged by the lens. Due to the combined action of both lenses, the image of the distant object is formed at the retina. As a result, the person can see the distant objects clearly. 2. Hypermetropia Hypermetropia is the defect of vision in which a person can see distant objects clearly but cannot see nearby objects. It is also called long-sightedness or far-sightedness. For such an eye, the near point is more than 25 cm. Causes of hypermetropia i. Due to the increase in the focal length of the eye lens or low converging power of the eye lens ii. Due to the contraction of the eye ball (a) An eye affected by hypermetropia (b) Correction of hypermetropia Convex lens Image Eye lens Eye lens Image Fig. Remedy: Hypermetropia can be corrected by using a convex lens of a suitable focal length. When a convex lens of suitable focal length is placed in front of the eye, the rays of the light coming from a nearby object are first converged by the convex lens and then by the eye lens. Due to the combined converging action of both the lenses, the image is formed at the retina. As a result, the person can see nearby objects clearly.

292 Oasis School Science and Technology - 10 Some other ways to treat defect of vision beside using spectacles Contact lenses The contact lenses are transparent curved lenses that are put on the eyes directly. It is usually used to correct visual defects. However, many 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 improves peripheral vision. Therefore, it is popular among athletes. It is used to correct shortsightedness, long-sightedness and other visual defects. Reasonable Fact 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. Spectacles produce distorted vision but lenses improves peripheral vision. Activity 7 Make a picture of contact lenses on A4-sized paper and enlist its advantages. Laser eye surgery for treatment of defect of vision [Laser-Assisted Insitu keratomileuses (LASIK)] People 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 tends to reshape the cornea itself so that light will refract and reach the cornea properly. As a result, defect of vision will be corrected. During LASIK surgery, a special type of cutting laser called excimer laser is used to precisely change the shape of imperfectly shaped cornea to improve vision. Fig: using contact lenses on eyes Fact File Contact lenses are so safe that only 1 out of 500 people get a serious eye infection! Fig: Procedure of laser eye surgery Fact File Laser surgery of eye can be completed within a couple of minutes.

Oasis School Science and Technology - 10 293 Reasonable Fact 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. Activity 8 Many people may be afraid of laser eye surgery. How would you convince them that it is not harmful technology? Reasonable Thinking Skill Reasonable Thinking Skill R T S 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. Mr. Ram needs to hold his newspaper very far away while reading it. What kind of defect of vision does he have? What should he do to correct it? Mr. Ram needs to hold his newspaper very far away while reading it. It means he cannot see object closer to the eye but can see if an object is far away from the eye. Therefore, he must be suffering from long-sightedness. He should wear a convex lens to correct this defect of vision. 3. Pooja is a taekwondo player. She prefers contact lenses instead of glasses. Why does she prefer it? Pooja 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. 4. Sarim was playing on the ground during lunchtime. When the bell rang, he ran toward the classroom. He found the classroom very dark but a few minutes later the class was bright for him again. He could see things properly. Explain why he found the room darker earlier but he could see just fine a few minutes later. When Sarim ran from the ground to the class suddenly, he found that the class was dark to see but it became brighter after a few minutes. The reason for this is an accommodation of the eye. The eye can adapt to see properly. When Sarim was playing in bright light, his pupil must have been small to prevent the excessive entry of light. When he suddenly entered the room, the pupil was still small, so very less light entered his eye. Therefore, he could not see properly. The room appeared dark to him.But after a few minutes, pupils grow larger automatically and more light entered through them. Then the room became brighter for him.

294 Oasis School Science and Technology - 10 5. 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. 6. Mahesh has difficulty distinguishing various shades of red and green. What defect of vision does he have? What is the cause of this defect? If Mahesh has difficulty distinguishing various shades of red and green, then he is suffering from red-green colour blindness. Colour blindness can be caused by: i. Heredity ii. Eye diseases iii. Alzheimer iv. Multiple sclerosis 7. Niroj was rearing a cow. He hit the cow with a stick. The stick broke and punctured his eye. His cornea was damaged. What kind of problem in a vision he could experience? How can he improve his vision? Niroj got a corneal injury from a stick while hitting a cow. Corneal injury can disturb vision. Some of them are: i. Tears flow regularly that can blur vision. ii. Scars can distort light and cause blur images. iii. Scars can block light from entering the eye. iv. Corneal injury can lead to eye cataracts and blindness. He can do the following things to improve his vision: i. He can use antibiotic drops to prevent infection. ii. Rinse the eye with clean water. iii. Blink several times. 8. 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 = 12.5 = 0.4 m = 40 centimetres Negative sign is not used because length cannot be negative. 9. Image distance for an eye is always constant. Why is it constant? How is it kept constant? Discuss. The image distance of an eye lens is always constant despite varying object distance so

Oasis School Science and Technology - 10 295 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 the ciliary muscles. 10. Convex lenses can converge parallel beams of light to make images. Under what condition it can make a virtual magnified image? Mention the application 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 watch makers to magnify small parts of the watch iv. to make simple microscopes for science labs. Exercises 1. Choose the best answer from the given alternatives. a. Where does the refracted light from the concave lens go? i. centre of curvature ii. optical centre iii. principal focus iv. to the centre of the lens b. Why is a human eye called an optical device? i. it is round and white ii. it has a pupil and retina iii. it has a lens and controls the light iv. it helps to see c. What is the power of the lens whose focal length is 2 meter? i. 2 D ii. 0.2 D iii. 0.5 D iv. 0.05 D d. What is the distance from the focus to the optical centre called? i. diameter of the lens ii. focal length iii. principle axis iv. focusing e. What is a real image? i. Image formed at 2F ii. image formed by dispersing rays. iii. image formed by converging rays iv. image formed if the refracted rays actually meet 2. Define the following terms with required examples. a. Lens b. Optical centre c. Principal axis d. Principal focus e. Focal length f. Centre of curvature g. Power of the lens h. Sclera i. Retina j. Choroid k. Pupil l. Cornea m. Iris n. Ciliary muscle o. Near point p. Accommodation of eye q. Far point r. Colour blindness s. Night blindness t. Eye cataract

296 Oasis School Science and Technology - 10 3. Answer the following questions in very short. a. Name the type of lens which forms a real image. b. What do you mean by myopia and hypermetropia? c. What is the geometrical centre of the lens called? d. Write the name of the muscle that holds the lens of the eye? e. What is it called if an eye has any kind of trouble seeing? f. Write the nature of the image formed by the concave lens and convex lens. g. Convex lens has made a virtual image. Where is the object placed? 4. Give reasons. a. Concave lenses can diverge parallel beams of light. b. Concave lens is called a diverging lens. c. Convex lens can converge parallel beams of light. d. Convex lens is called a converging lens. e. A concave lens is used to remove myopia. f. Far-sightedness can be removed by using a convex lens. g. Paper burns when sunlight is focused on a piece of paper with the help of a convex lens. h. Insufficient consumption of green vegetables and yellow fruits can cause night blindness in children. i. Colour blindness is caused if cone cells are malfunctioning. 5. Differentiate between the following. a. Convex lens and concave lens b. Real image and virtual image c. Night blindness and colour blindness d. Short-sightedness and long-sightedness e. Far point and near point 6. Answer the following questions in short. a. Write two main causes of hypermetropia. b. Write down the main causes of myopia. c. In which defect of vision, is a concave lens used? Why? d. If a man with a long-sighted eye wants to read a textbook, what should be the distance between the book and the lens? Give reason. e. What are the possible causes of eye cataracts? How can it be treated? f. What are the causes and remedies of night blindness? g. Write down two causes and remedies of short sight and long sight. h. Compare the spectacles with contact lenses. i. What is laser surgery? How does it help to treat defects of vision? j. Intraocular lens is used to treat various defects of vision in Nepal. Mention its benefits. 7. Answer the following long questions. a. What type of defect of vision is shown in the given diagram? Draw a labelled diagram for the correction of such defect.

Oasis School Science and Technology - 10 297 b. Focal length of a convex lens is 2cm. An object of height 1 cm is placed at a distance of 1 cm from the lens. Draw a ray diagram and write the nature of the image formed. c. What type of defect is found in the given diagram of the eye? State the cause of the defect and draw a diagram to show how this defect can be corrected. d. Write any two applications of a convex lens. A man uses a lens of power 1.5D. Find the focal length of that lens. (Ans: 0.66) e. Write any two utilities of a concave lens. Which type of lens is shown in the diagram? Calculate the power of the lens. (Ans: –50D) F F 2cm A f. Draw a clear ray diagram of the image formed when an object is placed 40cm away from a lens having a focal length of 20 cm. g. A student on the last bench of a class cannot read the letters written on the board. Which type of defect of the eye does s/he have? Draw a figure showing such type of defect of vision. Complete the given ray diagram. F Obejct O h. Draw a diagram of a short-sighted eye and also draw a diagram to show the correction of short sight. i. Draw a diagram of a long-sighted eye and also draw a diagram to show the correction of long sight. j. After examining the eye of a student, a doctor suggested spectacles of power 1.5D for him. Answer the following questions. i) What type of defect is there in his eye? ii) Draw a diagram to show the correction of this defect after using the spectacles. k. 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. l. Image distance of a convex lens changes when objects are placed in different places. Eyes also have convex lenses. What happens if the image distance of the eye lens changes continuously when we view objects at a varying distance? Do the image distance of our eye lens change? Why or why not? 8. Draw the schematic diagram of the eye and label the retina, lens and cornea. Image Retina

298 Oasis School Science and Technology - 10 Key terms and terminologies 1. Electric current : The flow of electrons in a definite direction is called electric current. 2. 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. 3. Alternating current : If the current changes direction after equal intervals of time, it is called alternating current (AC). 4. Direct current : If the current always flows in the same direction, it is called a direct current (DC). UNIT 11 CURRENT ELECTRICITY AND MAGNETISM Estimated teaching periods Theory 9 Practical 3 • 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 The Sequence of Curriculum Issued by CDC 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 demised on 25th August 1867. He was an excellent experimental physicist. He was honoured with the Royal medal, Copley medal, Rumford medal and Albert medal. About the Scientist Michael Faraday

Oasis School Science and Technology - 10 299 5. 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. 6. Solenoid : The coil of an insulated wire around a cylinder is called a solenoid. 7. Right hand grip rule : Maxwell's right hand grip rule for solenoid " If we grip the solenoid with our right hand so that fingers wraps the solenoid and the thumb is stretched out, then if the current is flowing in the direction of wrapped fingers then the thumb represents the direction of north pole. 8. Magnetic flux : The number of magnetic lines of force passing through a certain area of a conductor held perpendicularly is called magnetic flux. 9. Motor effect : When a current carrying conductor is placed perpendicular to the magnetic field, the conductor moves. This effect is called motor effect. 10. Electric motor : An electric motor is a device which converts electrical energy into mechanical energy. 11. Electromagnetic induction: The process of inducing current in a closed coil due to the relative motion between the magnet and the coil is called electromagnetic induction. 12. Generator : A generator, or dynamo, is a device which converts mechanical energy (or kinetic energy) into electrical energy. 13. Hydroelectricity : The electricity produced when turbine of generator is rotated by the flowing water is called hydroelectricity. 14. Wind energy : The energy produced by the fast-moving wind is called wind energy. 15. Thermal energy : The energy produced from the temperature of the heated substance is called thermal energy. 16. Transformer : A transformer is a device that converts high voltage AC to low voltage AC of the same frequency and vice versa. 17. 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." 18. Step up transformer : The transformer which converts low voltage AC to high voltage AC is called a step up transformer. 19. Step down transformer : The transformer which converts high voltage AC to low voltage AC is called a step down transformer. Introduction Electricity is a vital source of energy in the world we live in now. We use electricity in our homes, offices, factories, laboratories, and many other places. It is used to make heat and light, as well as to run machines, electronics, and other things. When two opposite poles of an electric source, such as a battery, are connected by a conductor, charges flow continuously through the conductor. Such continuous flow of charges produces a current, which is called electric current. The flow of electrons in a definite direction is called electric current. When current is passed through a resistance, it causes a heating effect. This fact was studied by James Prescott Joule in 1840 AD.

300 Oasis School Science and Technology - 10 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. Alternating current (AC) and Direct current (DC) If the current changes direction after equal intervals of time, it is called alternating current (AC). A generator or dynamo produces alternating current. The polarity of an alternating current is not fixed. Thus, due to the change of polarity in AC, it has non-zero frequency. Many devices such as air conditioner, fan, water pump, refrigerator etc. operates on AC. If the current always flows in the same direction, it is called a direct current (DC). A cell or a battery produces direct current because the current always flows in the same direction. The polarity of a direct current is fixed. It does not have frequency. Many electronic devices such as laptops, mobiles, torch lights etc. operates on DC. Differences between AC and DC S.N AC S.N. DC 1. The current which changes magnitude and polarity is called alternating current (AC). 1. The current which does not change magnitude and polarity is called direct current (DC). 2. It is produced by AC generator, dynamo, etc. 2. It is produced by cells, battery, DC generator, etc. 3. It can be used in a transformer to increase or decrease the voltage. 3. It cannot be used in a transformer. 4. It has non-zero frequency. 4. It has zero frequency. Current time graph The graphical representation of the change in current in a conductor against change in time is called current-time graph. If DC is flowing through the conductor, magnitude of current remains constant because DC does not change direction. So, Current-time graph is parallel to x axis as shown in figure I. If AC is flowing through the conductor, electrons keep changing direction back and forth continuously. As a result a wave like graph is formed as shown in figure II. The current increases from zero to certain ampere. Then it decreases to zero and beyond to negative values. The current will again increase from negative value to zero. Hence one oscillation is complete. Total number of oscillations per second is called frequency. Switch Cell Copper wire Bulb Electric circuit Fact File The frequency of AC is 50 Hz means that 50 cycles are made by the current in one second. Fact File Rectifier converts AC to DC to run mobile, computer, etc.