Light - GESC111 PDF

11 Light Y ou might have seen a beam of sunlight when it enters a room through a narrow opening or a hole. You may have also seen beams of light from the headlamps of scooters, cars and engines of trains [Fig. 11.1 (a)]. Similarly, a beam of light can be...

11 Light Y ou might have seen a beam of sunlight when it enters a room through a narrow opening or a hole. You may have also seen beams of light from the headlamps of scooters, cars and engines of trains [Fig. 11.1 (a)]. Similarly, a beam of light can be seen from a torch. Some of (a) Rail engine (b) Light house Fig. 11.1 Beams of light you may have seen a beam of searchlight 11.1 LIGHT TRAVELS ALONG A from a light house or from an airport STRAIGHT LINE tower [Fig. 11.1 (b)]. Boojho recalls an activity he performed What do these experiences suggest? in Class VI. In that activity he looked (a) (b) Fig. 11.2 Looking at a candle through a straight and a bent pipe LIGHT 123 Reprint 2024-25  Any polished or a shiny surface can act as a mirror. What happens when light falls on a mirror? You have learnt in Class VI that a mirror changes the direction of light that falls on it. This change of direction by a mirror is called reflection of light. Can you recall the activity in which you got the light of a torch reflected from a mirror? Let us perform a similar activity. Activity 11.1 Take a torch. Cover its glass with a chart paper which has three narrow slits as Fig. 11.3 Reflection of objects in water shown in Fig. 11.5. Spread a sheet of at a lighted candle first through a straight pipe and then through Paheli remembers the story of the lion and the a bent pipe (Fig. 11.2). Why was rabbit from the Panchtantra, in which the Boojho not able to see the candle rabbit fooled the lion by showing him his flame through a bent pipe? reflection in water (Fig. 11.4). This activity showed that light travels along straight lines. How can we change the path of light? Do you know, what happens when light falls on a polished or a shiny surface? 11.2 REFLECTION OF LIGHT One way to change the direction of light is to let it fall on a shiny surface. For example, a shining stainless steel plate or a shining steel spoon can change the direction of light. The surface of water can also act like a mirror and change the path of light. Have you ever seen the reflection of trees or buildings in water Fig. 11.4 Reflection of the lion in water (Fig. 11.3)? 124 SCIENCE Reprint 2024-25  Look into the mirror along the direction of the reflected light. Do you see the slits in the mirror? This is the image of the slits. This activity shows how light gets reflected from a plane mirror. Let us play around with the images formed in mirrors and know a little more about them. Activity 11.2 CAUTION Fig. 11.5 Reflection of light from a mirror Handle the lighted candle with care. chart paper on a smooth wooden board. It is better if this activity is performed Fix a plane mirror strip vertically on the in the presence of a teacher or an elder person. chart paper (Fig. 11.5). Now direct the beam of light on the mirror from the Place a lighted candle in front of a plane torch with slits. Place the torch in such mirror. Try to see the flame of the candle a way that its light is seen along the in the mirror. It appears as if a similar chart paper on the board. Now adjust candle is placed behind the mirror. The its position so that the light from the candle, which appears behind the torch strikes the plane mirror at an mirror, is the image of the candle formed angle (Fig. 11.5). by the mirror (Fig. 11.6). The candle Does the mirror change the direction itself is the object. of light that falls on it? Now move the Now move the candle to different torch slightly to either side. Do you find positions in front of the mirror. Observe any change in the direction of reflected the image in each case. light? Paheli wants to know, what makes things visible to us? Boojho thinks that objects are visible only when light reflected from them reaches our eyes. Do you agree with him? Fig. 11.6 Image of a candle in a plane mirror LIGHT 125 Reprint 2024-25  cannot be obtained on the screen in either case. Boojho noted in his notebook: Is What about the distance of the image it not surprising that my image is from mirror? Let us perform another of the same size as me whether activity. the mirror is small or large? Activity 11.3 Take a chess board. If a chess board is not available, draw on a chart paper 64 Was the image upright in each case? (8×8) squares of equal size. Draw a thick Did the flame appear on top of the candle line in the middle of the paper. Fix a as in the object? Such an image is called plane mirror vertically on this line. Place erect. An image formed by a plane any small object, such as a pencil mirror is erect and of the same size as sharpner, at the boundary of the third the object. square counting from the mirror Now place a vertical screen behind (Fig. 11.7). Note the position of the the mirror. Try to obtain the image of image. Now shift the object to the the candle on this screen. Can you get boundary of the fourth square. Again the image on the screen? Now place the note the position of the image. Did you screen in front of the mirror. Can you find any relation between the distance get the image on the screen now? You of the image from the mirror and that of will find that the image of the candle the object in front of it? Fig. 11.7 Locating image in a plane mirror Paheli made a note in her notebook: In a plane mirror the image is You will find that the image formed behind the mirror. It is erect, is at the same distance behind of the same size and is at the same the mirror as the object is in front distance from the mirror as the of it. Now verify this by placing object is in front of it. the object anywhere on the chart paper. 126 SCIENCE Reprint 2024-25 11.3 RIGHT OR LEFT! When you see your image in a plane mirror, is it exactly like you? Have you ever noticed that there is one interesting difference between you and your image in a mirror? Let us find out. Activity 11.4 Stand in front of a plane mirror and look at your image. Raise your left hand. Which hand does your image raise (Fig. 11.8)? Now touch your right ear. Which ear does your hand touch in your Fig. 11.9 An ambulance image? Observe carefully. You will find that in the mirror the ‘right’ appears ‘left’ mirror. How does it appear in the and the ‘left’ appears ‘right’. Note that mirror? only sides are interchanged; the image does not appear upside down. Now write down your name on a piece Boojho saw an ambulance on of paper and hold it in front of a plane the road. He was surprised to see that the word ‘AMBULANCE’ in front was written in a strange manner. Can you now understand why the word ‘AMBULANCE’ is written as in Fig. 11.9 ? When the driver of a vehicle ahead of an ambulance looks in her/his rear view mirror, she/he can read ‘AMBULANCE’ written on it and give way to it. It is the duty of every one of us to allow an ambulance to pass without blocking its way. You might have observed that in the side mirror of a scooter or a car the images of all the objects appear smaller Fig. 11.8 Left hand appears on the right side than the objects themselves. Have you in the image ever wondered why is it so? LIGHT 127 Reprint 2024-25 11.4 PLAYING WITH SPHERICAL Now look at your image using the MIRRORS inner side of the spoon. This time you Paheli and Boojho were waiting for their may find that your image is erect and dinner. Boojho lifted a stainless steel larger in size. If you increase the distance plate and saw his image in it. Oh! This of the spoon from your face, you may plate acts as a plane mirror. My image see your image inverted (Fig. 11.11). You is erect and is of the same size. Paheli can also compare the image of your pen saw her image using the back of a steel or pencil instead of your face. spoon. “Boojho look here! I can also see my erect image though it is smaller in size. This spoon also acts as a mirror of some kind”, said Paheli. You can also use a spoon or any curved shining surface to see your image. Activity 11.5 Take a stainless steel spoon. Bring the outer side of the spoon near your face and look into it. Do you see your image in it (Fig. 11.10)? Is this image different Fig. 11.11 Image from the inner side of a from what you see in a plane mirror? Is spoon this image erect? Is the size of the image The curved shining surface of a spoon the same, smaller or larger? acts as a mirror. The most common example of a curved mirror is a spherical mirror. If the reflecting surface of a spherical mirror is concave, it is called a concave mirror. If the reflecting surface is convex, then it is a convex mirror (Fig. 11.12). (a) (b) Fig. 11.10 Image from the outer side of a spoon Fig. 11.12 (a) A concave and (b) a convex mirror 128 SCIENCE Reprint 2024-25  Why are concave and convex mirrors called spherical mirrors? Take a rubber ball and cut a portion of it with a knife or a hack- saw blade [Fig. 11.13 (a)]. (Be careful. Ask an elder person to help you in cutting the ball). The inner surface of the cut ball is called concave and the outer surface is called convex (Fig. 11.13 (b)). Fig. 11.14 A concave mirror forms a real image of the sun Convex surface Take a concave mirror. Hold it facing the Sun. Try to get the light reflected by the mirror on a sheet of paper. Adjust the distance of the paper until you get a Concave surface sharp bright spot on it (Fig. 11.14). Hold (a) (b) the mirror and the sheet of paper steady Fig. 11.13 A spherical mirror is a part of for a few minutes. Does the paper start a sphere burning? The inner surface of a spoon acts like This bright spot is, in fact, the image a concave mirror, while its outer surface of the Sun. Notice that this image is acts like a convex mirror. formed on a screen. An image formed We know that the image of an object on a screen is called a real image. formed by a plane mirror cannot be Recollect that in Activity 11.2 the image obtained on a screen. Let us investigate formed by a plane mirror could not be if it is also true for the image formed by obtained on a screen. Such an image is a concave mirror. called a virtual image. Now let us try to obtain on the screen Activity 11.6 the image of a candle flame formed by a CAUTION concave mirror. You will conduct Activity 11.6 in the Activity 11.7 sunlight. Be careful, never look Fix a concave mirror on a stand (any directly towards the Sun or its image arrangement to keep the mirror steady as it may damage your eyes. You may would do) and place it on a table look at the image of the Sun when it (Fig. 11.15). Paste a piece of white paper is thrown on a screen or a wall. on a cardboard sheet (say about LIGHT 129 Reprint 2024-25  Fig. 11.15 Real images formed by a concave mirror 15 cm × 10 cm). This will act as a screen. obstruct the light from the candle falling Keep a lighted candle on the table at a on the mirror. Is this image real or distance of about 50 cm from the mirror. virtual? Is it of the same size as the flame? Try to obtain the image of the flame on Now move the candle towards the the screen. For this, move the screen till mirror and place it at different distances a sharp image of the flame is obtained. from it. In each case try to obtain the Make sure that, the screen does not image on the screen. Record your observation in Table 11.1. Is it possible to obtain the image on the screen when the candle is too close to the mirror (Fig. 11.16)? We see that the image formed by a concave mirror can be smaller or larger in size than the object. The image may also be real or virtual. Concave mirrors are used for many purposes. You might have seen doctors using concave mirrors for examining eyes, ears, nose and throat. Concave mirrors are also used by dentists to see an enlarged image of the teeth (Fig. 11.17). The reflectors of torches, Fig. 11.16 Virtual image formed by a concave headlights of cars and scooters are mirror concave in shape (Fig. 11.18). 130 SCIENCE Reprint 2024-25  if the bell is also a kind of spherical mirror. Can you recognise the type of the mirror? Note that the reflecting surface of the bell is convex. Activity 11.8 Repeat Activity 11.7 now with a convex mirror in place of a concave mirror (Fig. 11.19). Record your observations Fig. 11.17 A dentist examining a patient in a Table similar to Table 11.1. Boojho observed his image in the Could you get a real image at any shiny surface of the bell on his new distance of the object from the convex bicycle. He found that his image was erect and smaller in size. He wondered Concave surface Fig. 11.18 Reflector of a torch Fig. 11.19 Image formed by a convex mirror Table 11.1 Image formed by a concave mirror for object placed at different distances from it Distance of the object from Smaller/larger Character of the image the mirror than the object Inverted/ erect Real/virtual 50 cm … … 40 cm … … 30 cm 20 cm 10 cm … 5 cm LIGHT 131 Reprint 2024-25  Fig. 11.20 Convex mirror as side view mirror Fig. 11.21 A magnifying glass mirror? Did you get an image larger in which feel thinner in the middle than size than the object? at the edges are concave lenses Can you now recognise the mirrors [Fig. 11.22 (b)]. Notice that the lenses used as side mirrors in automobiles? are transparent and light can pass These are convex mirrors. Convex mirrors through them. can form images of objects spread over a large area. So, these help the drivers to see the traffic behind them (Fig. 11.20). 11.5 IMAGES FORMED BY LENSES (a) (b) You might have seen a magnifying glass. It is used to read very small print (Fig. 11.21). You might have also used it to observe the body parts of a Fig. 11.22 (a) A convex lens and (b) a concave cockroach or an earthworm. The lens magnifying glass is actually a type of a lens. Lenses are widely used in spectacles, Let us play with lenses. telescopes and microscopes. Try to add CAUTION a few more uses of lenses to this list. Get some lenses. Touch and feel It is dangerous to look through a lens them. Can you find some difference just at the Sun or a bright light. You by touching? Those lenses which feel should also be careful not to focus thicker in the middle than at the edges sunlight with a convex lens on any are convex lenses [Fig. 11.22 (a)]. Those part of your body. 132 SCIENCE Reprint 2024-25 Activity 11.9 A convex lens converges (bends Take a convex lens or magnifying glass. inward) the light generally falling on it Put it in the path of sunrays. Place a [Fig. 11.24 (a)]. Therefore, it is called a sheet of paper as shown (Fig. 11.23). converging lens. On the other hand, a Adjust the distance between the lens and concave lens diverges (bends outward) the paper till you get a bright spot on the light and is called a diverging lens the paper. Hold the lens and the paper [Fig. 11.24 (b)]. in this position for a few minutes. Does the paper begin to burn? Now replace the convex lens with a (a) concave lens. Do you see a bright spot (b) Fig. 11.24 of the candle on a paper screen placed on the other side of the lens. You may have to move the screen towards or away Fig. 11.23 Real image of the sun by a convex lens from the lens to get a sharp image of the flame. What kind of image did you get? Is it real or virtual? on the paper this time, too? Why are Now vary the distance of the candle you not getting a bright spot this time? from the lens [Fig. 11.25 (b)]. Try to We have seen in the case of mirrors obtain the image of the candle flame that for different positions of the object every time on the paper screen by the nature and size of the image change. moving it. Record your observations as Is it true for lenses also? you did in Activity 11.7 for the concave Let us find out. mirror. Activity 11.10 Take a convex lens and fix it on a stand It means that we can see the as you did with the concave mirror. Place image formed by a lens from the it on a table. Place a lighted candle at a side opposite to that of the object. distance of about 50 cm from the lens [Fig. 11.25 (a)]. Try to obtain the image LIGHT 133 Reprint 2024-25  (a) (b) Fig. 11.25 Image by a convex lens for object placed at different distance from it Fig. 11.26 Virtual image formed by the convex lens Fig. 11.27 Image formed by a concave lens Did you get in any position of the is always virtual, erect and smaller in object an image which was erect and size than the object (Fig. 11.27). magnified (Fig. 11.26). Could this image be obtained on a screen? Is the image 11.6 SUNLIGHT — WHITE OR real or virtual? This is how a convex lens COLOURED? is used as a magnifying glass. Have you ever seen a rainbow in the In a similar fashion study the images sky? You might have noticed that it formed by a concave lens. You will find appears usually after the rain when the that the image formed by a concave lens Sun is low in the sky. The rainbow is 134 SCIENCE Reprint 2024-25  Fig. 11.29 A CD placed in sun Fig. 11.28 A rainbow You might have seen that when you seen as a large arc in the sky with many blow soap bubbles, they appear colours (Fig. 11.28). colourful. Similarly, when light is How many colours are present in a reflected from the surface of a rainbow? When observed carefully, there Compact Disk (CD), you see many are seven colours in a rainbow, though colours (Fig. 11.29). it may not be easy to distinguish all of On the basis of these experiences, them. These are — red, orange, yellow, could we say that the sunlight is a green, blue, indigo and violet. mixture of different colours? Let us investigate. Activity 11.11 Take a glass prism. Allow a narrow beam Does this mean that the of sunlight through a small hole in the white light consists of window of a dark room to fall on one seven colours? face of the prism. Let the light coming out of the other face of the prism fall on Paheli wants to tell you that you can see a rainbow only when your back is towards the sun. Fig. 11.30 A prism splits a beam of sunlight into seven colours LIGHT 135 Reprint 2024-25 a white sheet of paper or on a white wall. seven segments. Paint the seven What do you observe? Do you see colours rainbow colours on these segments similar to those in a rainbow as shown in Fig. 11.31 (a). You can (Fig. 11.30)? This shows that the also paste, coloured papers on sunlight consists of seven colours. The these segments. Make a small hole sunlight is said to be white light. This at the centre of the disc. Fix the means that the white light consists of disc loosely on the tip of a refill of a seven colours. Try to identify these ball pen. Ensure that the disc rotates colours and write their names in your freely [Fig. 11.31 (a)]. Rotate the disc notebook. in the daylight. When the disc is Can we mix these colours to get white rotated fast, the colours get mixed light? Let us try. together and the disc appears to be whitish [Fig. 11.31 (b)]. Such Activity 11.12 a disc is popularly known as Newton’s Take a circular cardboard disc of about disc. 10 cm diameter. Divide this disc into Fig. 11.31 (a) A disc with seven colours (b) It appears white on rotating Paheli has a brilliant idea! She has prepared a small top with a small circular disc with seven rainbow colours painted on it (Fig. 11.32). When the top rotates it appears nearly white. Fig. 11.32 A top with seven colours 136 SCIENCE Reprint 2024-25  Keywords Concave lens Magnified image Rear view mirror Concave mirror Magnifying glass Side mirror Convex lens Prism Spherical mirror Convex mirror Rainbow Virtual image Erect image Real image What you have learnt Light travels along straight lines. Any polished or a shining surface acts as a mirror. An image which can be obtained on a screen is called a real image. An image which cannot be obtained on a screen is called a virtual image. The image formed by a plane mirror is erect. It is virtual and is of the same size as the object. The image is at the same distance behind the mirror as the object is in front of it. In an image formed by a mirror, the left side of the object is seen on the right side in the image, and right side of the object appears to be on the left side in the image. A concave mirror can form a real and inverted image. When the object is placed very close to the mirror, the image formed is virtual, erect and magnified. Image formed by a convex mirror is erect, virtual and smaller in size than the object. A convex lens can form real and inverted image. When the object is placed very close to the lens, the image formed is virtual, erect and magnified. When used to see objects magnified, the convex lens is called a magnifying glass. A concave lens always forms erect, virtual and smaller image than the object. White light is composed of seven colours. LIGHT 137 Reprint 2024-25  Exercises 1. Fill in the blanks: (a) An image that cannot be obtained on a screen is called ____________. (b) Image formed by a convex __________ is always virtual and smaller in size. (c) An image formed by a __________ mirror is always of the same size as that of the object. (d) An image which can be obtained on a screen is called a _________ image. (e) An image formed by a concave ___________ cannot be obtained on a screen. 2. Mark ‘T’ if the statement is true and ‘F’ if it is false: (a) We can obtain an enlarged and erect image by a convex mirror. (T/F) (b) A concave lens always form a virtual image. (T/F) (c) We can obtain a real, enlarged and inverted image by a concave mirror. (T/F) (d) A real image cannot be obtained on a screen. (T/F) (e) A concave mirror always form a real image. (T/F) 3. Match the items given in Column I with one or more items of Column II. Column I Column II (a) A plane mirror (i) Used as a magnifying glass. (b) A convex mirror (ii) Can form image of objects spread over a large area. (c) A convex lens (iii) Used by dentists to see enlarged image of teeth. (d) A concave mirror (iv) The image is always inverted and magnified. (e) A concave lens (v) The image is erect and of the same size as the object. (vi) The image is erect and smaller in size than the object. 4. State the characteristics of the image formed by a plane mirror. 5. Find out the letters of English alphabet or any other language known to you in which the image formed in a plane mirror appears exactly like the letter itself. Discuss your findings. 138 SCIENCE Reprint 2024-25  6. What is a virtual image? Give one situation where a virtual image is formed. 7. State two differences between a convex and a concave lens. 8. Give one use each of a concave and a convex mirror. 9. Which type of mirror can form a real image? 10. Which type of lens forms always a virtual image? Choose the correct option in questions 11–13 11. A virtual image larger than the object can be produced by a (i) concave lens (ii) concave mirror (iii) convex mirror (iv) plane mirror 12. David is observing his image in a plane mirror. The distance between the mirror and his image is 4 m. If he moves 1 m towards the mirror, then the distance between David and his image will be (i) 3m (ii) 5m (iii) 6m (iv) 8m 13. The rear view mirror of a car is a plane mirror. A driver is reversing his car at a speed of 2 m/s. The driver sees in his rear view mirror the image of a truck parked behind his car. The speed at which the image of the truck appears to approach the driver will be (i) 1 m/s (ii) 2 m/s (iii) 4 m/s (iv) 8 m/s Extended Learning — Activities and Projects 1. Play with a mirror Write your name with a sketch pen on a thin sheet of paper, polythene or glass. Read your name on the sheet while standing in front of a plane mirror. Now look at your image in the mirror. 2. A burning candle in water Take a shoe box, open on one side. Place a small lighted candle in it. Place a clear glass sheet (roughly 25 cm × 25 cm) infront of this candle (Fig. 11.33). Try to locate the image of the candle behind Fig. 11.33 Candle burning in water LIGHT 139 Reprint 2024-25  the glass sheet. Place a glass of water at its position. Ask your friends to look at the image of the candle through the sheet of glass. Ensure that candle is not visible to your friends. Your friends will be surprised to see the candle burning in water. Try to explain the reason. 3. Make a rainbow Try to make your own rainbow. You can try this project in the morning or in the evening. Stand with your back towards the Sun. Take a hosepipe or a water pipe used in the garden. Make a fine spray in front of you. You can see different colours of rainbow in the spray. 4. Visit a laughing gallery in some science centre or a science park or a village mela. You will find some large mirrors there. You can see your distorted and funny images in these mirrors. Try to find out the kind of mirrors used there. 5. Visit a nearby hospital. You can also visit the clinic of an ENT specialist, or a dentist. Request the doctor to show you the mirrors used for examining ear, nose, throat and teeth. Can you recognise the kind of mirror used in these instruments? 6. Role play Here is a game that a group of children can play. One child will be chosen to act as object and another will act as the image of the object. The object and the image will sit opposite to each other. The object will make movements, such as raising a hand, touching an ear etc. The image will have to make the correct movement following the movement of the object. The rest of the group will watch the movements of the image. If the image fails to make the correct movement, she/he will be retired. Another child will take her/his place and the game will continue. A scoring scheme can be introduced. The group that scores the maximum will be declared the winner. 140 SCIENCE Reprint 2024-25  Did you know? The mirrors can be used as weapons. Archimedes, a Greek scientist, is said to have done just that more than two thousand years ago. When the Romans attacked Syracuse, a coastal city-state in Greece, Archimedes used mirrors arranged as shown in Fig. 11.34. The mirrors could be moved in any direction. They were positioned such that they reflected the sunlight on the Roman soldiers. The soldiers were dazzled by the sunlight. They did not know what was happening. They got confused and ran away. This was an example of triumph of ideas over military might. Fig. 11.34 Archimedes mirrors LIGHT 141 Reprint 2024-25