Observe the picture given in Fig. 11.1 carefully. Fig. 11.1 A patch of light is obtained at B, when the torch is lighted as shown. Which of the following is kept at position A to get this patch of light?
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- (a)A wooden board
- (b)A glass sheet
- (c)A mirror
- (d)A sheet of white paper, 2. A student observes a tree given in Fig. 11.2 through a pin hole camera. Which of the diagrams given in Fig. 11.3 (a) to (d), depicts the image seen by her correctly? Fig. 11.2 L IGHT 65 (a) (b) (c) (d) Fig. 11.3
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Four students A, B, C and D looked through pipes of different shapes to see a candle flame as shown in Fig. 11.4. A B C D Fig. 11.4 Who will be able to see the candle flame clearly? 66 E XEMPLAR PROBLEMS
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Which of the following is/are not always necessary to observe a shadow?
- (a)Sun
- (b)Screen
- (c)Source of light
- (d)Opaque object
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Paheli observed the shadow of a tree at 8:00 a.m., 12:00 noon and 3:00 p.m. Which of the following statements is closest to her observation about the shape and size of the shadow?
- (a)The shape of the shadow of the tree changes but the size remains the same.
- (b)The size of the shadow of the tree changes but the shape remains the same.
- (c)Both the size and shape of the shadow of the tree change.
- (d)Neither the shape nor the size of the shadow changes.
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(c) Both the size and shape of the shadow of the tree change.
Which of the following can never form a circular shadow?
- (a)A ball
- (b)A flat disc
- (c)A shoe box
- (d)An ice cream cone
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Two students while sitting across a table looked down on to its top surface. They noticed that they could see their own and each other’s image. The table top is likely to be made of:
- (a)unpolished wood
- (b)red stone
- (c)glass sheet
- (d)wood top covered with cloth L IGHT 67
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You have 3 opaque strips with very small holes of different shapes as shown in Fig. 11.5. If you obtain an image of the sun on a wall through these holes, will the image formed by these holes be the same or different? Fig. 11.5
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The image obtained will be the same in all the three cases.
Observe the picture given in Fig. 11.6. A sheet of some material is placed at position ‘P’, still the patch of light is obtained on the screen. What is the type of material of this sheet? Screen Mirror Source of light Fig. 11.6 68 E XEMPLAR PROBLEMS
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A sheet of transparent material is placed at ‘P’.
Three torches A, B and C shown in Fig. 11.7 are switched on one by one. The light from which of the torches will not form a shadow of the ball on the screen. Fig. 11.7
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The light of torch at position C will not form a shadow of the ball on the screen.
Look at the figure given in Fig. 11.8. Fig. 11.8 L IGHT 69 Will there be any difference in the shadow formed on the screen in A and B.
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Correct the following statements.
- (i)The colour of the shadow of an object depends on its colour of the object.
- (ii)Transparent objects allow light to pass through them partially.
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(i) The colour of the shadow of an object depends on its colour of the object.
(ii) Transparent objects allow light to pass through them partially.
Suggest a situation where we obtain more than one shadow of an object at a time.
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We can obtain more than one shadow of an object if light from more than one source falls on it. [For example during a match being played in a stadium, multiple shadows of players are seen].
On a sunny day, does a bird or an aeroplane flying high in the sky cast its shadow on the ground? Under what circumstances can we see their shadow on the ground?
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No. Shadow of the bird can only be seen when the bird is flying very low close to the ground.
You are given a transparent glass sheet. Suggest any two ways to make it translucent without breaking it.
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(i) By applying oil, grease, butter on it or pasting a butter paper on it. (ii) Grinding (rubbing) the surface of the glass by any abrasive material. 130 E XEMPLAR PROBLEMS
A torch is placed at two different positions A and B, one by one, as shown in Fig. 11.9. Fig. 11.9 70 E XEMPLAR PROBLEMS The shape of the shadow obtained in two positions is shown in Fig. 11.10. Fig. 11.10 Match the position of the torch and shape of the shadow of the ball.
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A student covered a torch with red cellophane sheet to obtain red light. Using the red light she obtains a shadow of an opaque object. She repeats this activity with green and blue light. Will the colour of the light affect the shadow ? Explain.
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The colour of light will not affect the shadow, because shadow is the dark patch formed when an object obstructs the path of light and hence no light reaches in the shadow region.
Is air around us always transparent? Discuss.
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Air around us is transparent but when thick smoke, thick clouds, etc. are present in the air it does not remain transparent.
Three identical towels of red, blue and green colour are hanging on a clothes line in the sun. What would be the colour of shadows of these towels?
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The colour of shadows of all three towels will be the same.
Using a pinhole camera a student observes the image of two of his friends, standing in sunlight, wearing yellow and red shirt respectively. What will be the colours of the shirts in the image?
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The colours of the image of the shirts will be the same as the colour of the shirt.
In Fig. 11.11, a flower made of thick coloured paper has been pasted on the transparent glass sheet. What will be the shape and colour of shadow seen on the screen? Fig. 11.11 L IGHT 71
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The shadow formed will be dark and of the shape of the flower along with the stalk. LONG ANSWER QUESTIONS
A football match is being played at night in a stadium with flood lights ON. You can see the shadow of a football kept at the ground but cannot see its shadow when it is kicked high in the air. Explain.
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We can see the shadow of football lying on the ground because the ground acts as a screen for it. However, when the football is kicked high, the ground, which is acting as a screen is away from the football, hence no shadow of the football will be formed on the ground.
A student had a ball, a screen and a torch in working condition. He tried to form a shadow of the ball on the screen by placing them at different positions. Sometimes the shadow was not obtained. Explain.
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Some of the reason can be (i) The screen is away from the ball. (ii) The beam of light from the torch is falling parallel to the screen on the ball. (iii) The torch is kept away from the ball.
A sheet of plywood, a piece of muslin cloth and that of a transparent glass, all of the same size and shape were placed at A one by one in the arrangement shown in Fig. 11.12. Will the shadow be formed in each case. If yes, how will the shadow on the screen be different in each case? Give reasons for your answer. Fig. 11.12
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Shadow will not be formed in each case. Shadow will be formed by the sheet of plywood and the piece of muslin cloth. The sheet of plywood will form a dark shadow as it blocks the path of light completely. The piece of muslin cloth will form a lighter shadow as it allow light to pass through it partially. ANSWERS 131