Welcome dear students! Today we are going to learn about Light from Class 7 Science.
You 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. In Figure 11.1 part a, we see a rail engine with its headlight beam, and in part b, we see a lighthouse sending out a powerful beam of light. Similarly, a beam of light can be seen from a torch. Some of you may have seen a beam of searchlight from a lighthouse or from an airport tower. What do these experiences suggest? They suggest that light travels in a specific path. Let us explore this further.
Boojho recalls an activity he performed in Class six. In that activity, he looked at a lighted candle first through a straight pipe and then through a bent pipe, as shown in Figure 11.2. Figure 11.2 part a shows a straight pipe where the candle flame is clearly visible. Part b shows a bent pipe where the flame cannot be seen. Why was Boojho not able to see the candle flame through a bent pipe? 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?
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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? Figure 11.3 shows the reflection of objects in water. Paheli remembers the story of the lion and the rabbit from the Panchatantra, in which the rabbit fooled the lion by showing him his reflection in water. Figure 11.4 illustrates this scene, showing the lion looking at his own reflection in a water body. Any polished or a shiny surface can act as a mirror. What happens when light falls on a mirror? You have learnt in Class six 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 shown in Figure 11.5. Spread a sheet of chart paper on a smooth wooden board. Fix a plane mirror strip vertically on the chart paper. Now direct the beam of light on the mirror from the torch with slits. Place the torch in such a way that its light is seen along the chart paper on the board. Now adjust its position so that the light from the torch strikes the plane mirror at an angle. Does the mirror change the direction of light that falls on it? Now move the torch slightly to either side. Do you find any change in the direction of reflected 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? 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.
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Activity 11.2: Caution. Handle the lighted candle with care. It is better if this activity is performed in the presence of a teacher or an elder person. Place a lighted candle in front of a plane mirror. Try to see the flame of the candle in the mirror. It appears as if a similar candle is placed behind the mirror. The candle which appears behind the mirror is the image of the candle formed by the mirror, as shown in Figure 11.6. The candle itself is the object. Now move the candle to different positions in front of the mirror. Observe the image in each case. Boojho noted in his notebook that it is not surprising that his image is of the same size as him whether the mirror is small or large. Was the image upright in each case? Did the flame appear on top of the candle as in the object? Such an image is called erect. An image formed by a plane mirror is erect and of the same size as the object. Now place a vertical screen behind the mirror. Try to obtain the image of the candle on this screen. Can you get the image on the screen? Now place the screen in front of the mirror. Can you get the image on the screen now? You will find that the image of the candle cannot be obtained on the screen in either case. What about the distance of the image from mirror? Let us perform another activity.
Activity 11.3: Take a chess board. If a chess board is not available, draw on a chart paper sixty four squares of equal size in an eight by eight grid. Draw a thick line in the middle of the paper. Fix a plane mirror vertically on this line. Place any small object, such as a pencil sharpener, at the boundary of the third square counting from the mirror, as shown in Figure 11.7. Note the position of the image. Now shift the object to the boundary of the fourth square. Again note the position of the image. Did you find any relation between the distance of the image from the mirror and that of the object in front of it? Paheli made a note in her notebook that in a plane mirror the image is formed behind the mirror. It is erect, of the same size and is at the same distance from the mirror as the object is in front of it. You will find that the image is at the same distance behind the mirror as the object is in front of it. Now verify this by placing the object anywhere on the chart paper.
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Section 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? Figure 11.8 shows that the left hand appears on the right side in the image. Now touch your right ear. Which ear does your hand touch in your image? Observe carefully. You will find that in the mirror the right appears left and the left appears right. Note that only sides are interchanged; the image does not appear upside down. Now write down your name on a piece of paper and hold it in front of a plane mirror. How does it appear in the mirror? Boojho saw an ambulance on the road. He was surprised to see that the word AMBULANCE in front was written in a strange manner, as shown in Figure 11.9. Can you now understand why the word AMBULANCE is written in reverse? When the driver of a vehicle ahead of an ambulance looks in her or his rear view mirror, she or 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 than the objects themselves. Have you ever wondered why is it so?
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Section 11.4: Playing with Spherical Mirrors. Paheli and Boojho were waiting for their dinner. Boojho lifted a stainless steel plate and saw his image in it. He noticed that the plate acts as a plane mirror, and his image is erect and of the same size. Paheli saw her image using the back of a steel spoon. She said that she can also see her erect image though it is smaller in size, and that the spoon acts as a mirror of some kind. 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, as shown in Figure 11.10? Is this image different from what you see in a plane mirror? Is this image erect? Is the size of the image the same, smaller or larger? Now look at your image using the inner side of the spoon. This time you may find that your image is erect and larger in size. If you increase the distance of the spoon from your face, you may see your image inverted, as shown in Figure 11.11. You can also compare the image of your pen or pencil instead of your face. The curved shining surface of a spoon 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 called a convex mirror, as shown in Figure 11.12 parts a and b. 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, as shown in Figure 11.13 part 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, as shown in Figure 11.13 part b. The inner surface of a spoon acts like a concave mirror, while its outer surface acts like a convex mirror.
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We know that the image of an object formed by a plane mirror cannot be obtained on a screen. Let us investigate if it is also true for the image formed by a concave mirror.
Activity 11.6: Caution. You will conduct Activity 11.6 in the sunlight. Be careful, never look directly towards the Sun or its image as it may damage your eyes. You may look at the image of the Sun when it is thrown on a screen or a wall. 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 sharp bright spot on it, as shown in Figure 11.14. Hold the mirror and the sheet of paper steady for a few minutes. Does the paper start burning? This bright spot is, in fact, the image of the Sun. Notice that this image is formed on a screen. An image formed on a screen is called a real image. Recollect that in Activity 11.2 the image formed by a plane mirror could not be obtained on a screen. Such an image is called a virtual image. Now let us try to obtain on the screen the image of a candle flame formed by a concave mirror.
Activity 11.7: Fix a concave mirror on a stand and place it on a table, as shown in Figure 11.15. Paste a piece of white paper on a cardboard sheet. This will act as a screen. Keep a lighted candle on the table at a distance of about fifty centimeters from the mirror. Try to obtain the image of the flame on the screen. For this, move the screen till a sharp image of the flame is obtained. Make sure that the screen does not obstruct the light from the candle falling on the mirror. Is this image real or virtual? Is it of the same size as the flame? Now move the candle towards the mirror and place it at different distances from it. In each case try to obtain the 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, as shown in Figure 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, as shown in Figure 11.17. The reflectors of torches, headlights of cars and scooters are concave in shape, as shown in Figure 11.18.
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Boojho observed his image in the shiny surface of the bell on his new bicycle. He found that his image was erect and smaller in size. He wondered 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, as shown in Figure 11.19. Record your observations in a table similar to Table 11.1. Could you get a real image at any distance of the object from the convex mirror? Did you get an image larger in size than the object? Can you now recognise the mirrors used as side mirrors in automobiles? These are convex mirrors. Convex mirrors can form images of objects spread over a large area. So, these help the drivers to see the traffic behind them, as shown in Figure 11.20.
Section 11.5: Images Formed by Lenses. You might have seen a magnifying glass, as shown in Figure 11.21. It is used to read very small print. You might have also used it to observe the body parts of a cockroach or an earthworm. The magnifying glass is actually a type of a lens. Lenses are widely used in spectacles, telescopes and microscopes. Try to add a few more uses of lenses to this list. Get some lenses. Touch and feel them. Can you find some difference just by touching? Those lenses which feel thicker in the middle than at the edges are convex lenses, as shown in Figure 11.22 part a. Those which feel thinner in the middle than at the edges are concave lenses, as shown in Figure 11.22 part b. Notice that the lenses are transparent and light can pass through them. A convex lens converges, or bends inward, the light generally falling on it, as shown in Figure 11.24 part a. Therefore, it is called a converging lens. On the other hand, a concave lens diverges, or bends outward, the light and is called a diverging lens, as shown in Figure 11.24 part b.
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Caution: It is dangerous to look through a lens at the Sun or a bright light. You should also be careful not to focus sunlight with a convex lens on any part of your body.
Activity 11.9: Take a convex lens or magnifying glass. Put it in the path of sunrays. Place a sheet of paper as shown in Figure 11.23. Adjust the distance between the lens and the paper till you get a bright spot on the paper. Hold the lens and the paper in this position for a few minutes. Does the paper begin to burn? Now replace the convex lens with a concave lens. Do you see a bright spot on the paper this time, too? Why are you not getting a bright spot this time? We have seen in the case of mirrors that for different positions of the object the nature and size of the image change. Is it true for lenses also? Let us find out.
Activity 11.10: Take a convex lens and fix it on a stand as you did with the concave mirror. Place it on a table. Place a lighted candle at a distance of about fifty centimeters from the lens, as shown in Figure 11.25 part a. Try to obtain the image 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 from the lens to get a sharp image of the flame. What kind of image did you get? Is it real or virtual? Now vary the distance of the candle from the lens, as shown in Figure 11.25 part b. Try to obtain the image of the candle flame every time on the paper screen by moving it. Record your observations as you did in Activity 11.7 for the concave mirror. It means that we can see the image formed by a lens from the side opposite to that of the object. Did you get in any position of the object an image which was erect and magnified, as shown in Figure 11.26? Could this image be obtained on a screen? Is the image real or virtual? This is how a convex lens is used as a magnifying glass. In a similar fashion study the images formed by a concave lens. You will find that the image formed by a concave lens is always virtual, erect and smaller in size than the object, as shown in Figure 11.27.
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Section 11.6: Sunlight White or Coloured? Have you ever seen a rainbow in the sky? You might have noticed that it appears usually after the rain when the Sun is low in the sky. The rainbow is seen as a large arc in the sky with many colours, as shown in Figure 11.28. How many colours are present in a rainbow? When observed carefully, there are seven colours in a rainbow, though it may not be easy to distinguish all of them. These are red, orange, yellow, green, blue, indigo and violet. Does this mean that the white light consists of seven colours? You might have seen that when you blow soap bubbles, they appear colourful. Similarly, when light is reflected from the surface of a Compact Disk, you see many colours, as shown in Figure 11.29. On the basis of these experiences, could we say that the sunlight is a mixture of different colours? Let us investigate.
Activity 11.11: Take a glass prism. Allow a narrow beam of sunlight through a small hole in the window of a dark room to fall on one face of the prism. Let the light coming out of the other face of the prism fall on a white sheet of paper or on a white wall. What do you observe? Do you see colours similar to those in a rainbow, as shown in Figure 11.30? This shows that the sunlight consists of seven colours. The sunlight is said to be white light. This means that the white light consists of seven colours. Try to identify these colours and write their names in your notebook. Can we mix these colours to get white light? Let us try.
Activity 11.12: Take a circular cardboard disc of about ten centimeters diameter. Divide this disc into seven segments. Paint the seven rainbow colours on these segments as shown in Figure 11.31 part a. You can also paste coloured papers on these segments. Make a small hole at the centre of the disc. Fix the disc loosely on the tip of a refill of a ball pen. Ensure that the disc rotates freely, as shown in Figure 11.31 part a. Rotate the disc in the daylight. When the disc is rotated fast, the colours get mixed together and the disc appears to be whitish, as shown in Figure 11.31 part b. Such a disc is popularly known as Newtons disc. Paheli has a brilliant idea. She has prepared a small top with a small circular disc with seven rainbow colours painted on it, as shown in Figure 11.32. When the top rotates it appears nearly white.
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Let us review the keywords from this chapter. Concave lens, concave mirror, convex lens, convex mirror, erect image, magnified image, magnifying glass, prism, rainbow, real image, rear view mirror, side mirror, spherical mirror, virtual image.
Now let us go over 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.
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Now let us solve the exercises together. Exercise 1 asks to fill in the blanks. Part a: An image that cannot be obtained on a screen is called a virtual image. Part b: Image formed by a convex mirror is always virtual and smaller in size. Part c: An image formed by a plane mirror is always of the same size as that of the object. Part d: An image which can be obtained on a screen is called a real image. Part e: An image formed by a concave lens cannot be obtained on a screen.
Exercise 2 asks to mark true or false. Part a: We can obtain an enlarged and erect image by a convex mirror. This is false. Part b: A concave lens always form a virtual image. This is true. Part c: We can obtain a real, enlarged and inverted image by a concave mirror. This is true. Part d: A real image cannot be obtained on a screen. This is false. Part e: A concave mirror always form a real image. This is false.
Exercise 3 asks to match items. A plane mirror matches with the image is erect and of the same size as the object. A convex mirror matches with can form image of objects spread over a large area. A convex lens matches with used as a magnifying glass. A concave mirror matches with used by dentists to see enlarged image of teeth. A concave lens matches with the image is erect and smaller in size than the object.
Exercise 4 asks to state the characteristics of the image formed by a plane mirror. The image formed by a plane mirror is erect, virtual, of the same size as the object, and located at the same distance behind the mirror as the object is in front of it. Also, the left and right sides are interchanged.
Exercise 5 asks to find out letters of the English alphabet where the image in a plane mirror appears exactly like the letter itself. The letters are A, H, I, M, O, T, U, V, W, X, and Y. These letters have vertical symmetry, so their mirror images look identical to the original.
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Exercise 6 asks what a virtual image is and to give one situation where it is formed. A virtual image is an image that cannot be obtained on a screen. It is formed when light rays appear to diverge from a point behind the mirror or lens. One situation is the image formed by a plane mirror when you look at yourself.
Exercise 7 asks to state two differences between a convex and a concave lens. First, a convex lens is thicker in the middle and thinner at the edges, while a concave lens is thinner in the middle and thicker at the edges. Second, a convex lens converges light rays, whereas a concave lens diverges light rays.
Exercise 8 asks to give one use each of a concave and a convex mirror. A concave mirror is used by dentists to see an enlarged image of teeth. A convex mirror is used as a side view mirror in vehicles to see a wider area of traffic behind.
Exercise 9 asks which type of mirror can form a real image. A concave mirror can form a real image.
Exercise 10 asks which type of lens forms always a virtual image. A concave lens always forms a virtual image.
Exercise 11 asks which device produces a virtual image larger than the object. The correct option is a concave mirror.
Exercise 12 is a numerical problem. David is observing his image in a plane mirror. The distance between the mirror and his image is four meters. If he moves one meter towards the mirror, then the distance between David and his image will be six meters. This is because initially he was four meters from the mirror, so his image was four meters behind it, making the total distance eight meters. After moving one meter closer, he is three meters from the mirror, and his image is three meters behind it, making the total distance six meters.
Exercise 13 asks about the speed of the image of a truck in a rear view mirror. The rear view mirror of a car is a plane mirror. A driver is reversing his car at a speed of two meters per second. The speed at which the image of the truck appears to approach the driver will be four meters per second. This happens because both the car and the image move towards each other at the same relative speed.
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Now let us look at the extended learning activities and projects. First, 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. You will see your name reversed. Second, 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 twenty five by twenty five centimeters in front of this candle. Try to locate the image of the candle behind 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 the candle is not visible to your friends. Your friends will be surprised to see the candle burning in water. This happens because the glass reflects the candle light, creating an illusion. Third, make a rainbow. Try to make your own rainbow in the morning or 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. Fourth, 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. They are usually curved spherical mirrors. Fifth, visit a nearby hospital or the clinic of an ear, nose, throat specialist, or a dentist. Request the doctor to show you the mirrors used for examining ear, nose, throat and teeth. You will find they use concave mirrors to get magnified images. Sixth, role play. One child acts as object and another acts as the image. They sit opposite to each other. The object makes movements like raising a hand or touching an ear. The image must make the correct mirrored movement. If the image fails, she or he is retired. This game helps understand lateral inversion.
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Finally, let us learn an interesting historical fact. Did you know that 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 Figure 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.
Thank you for listening! Keep revising and practicing. Goodbye! [CHAPTER_COMPLETE]