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NCERT Class 7 Science Chapter 11 Light: Shadows and Reflections
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Light: Shadows and Reflections
Chapter: 11
Page No – 156
Table 11.1: Light through different materials.
| Material | Transparent/ Translucent/ Opaque | Light will pass fully/partially/not at all | |
| My prediction | My observation | ||
| Cardboard | |||
| Paper | |||
| Glass | |||
| Tracing paper | |||
| Thick cloth | |||
| Plastic sheet | |||
| Frosted glass | |||
Ans:
| Material | Transparent/ Translucent/ Opaque | Light will pass fully/partially/not at all | |
| My prediction | My observation | ||
| Cardboard | Opaque | Light will not pass | Light did not pass |
| Paper | Translucent/Opaque | Light will partially not pass | Light passed a little/ not at all |
| Glass | Transparent | Light will fully pass | Light fully passed |
| Tracing paper | Translucent | Light will partially pass | Light partially passed |
| Thick cloth | Opaque | Light will not pass | Light did not pass |
| Plastic sheet | Transparent/Translucent (depends on type) | Light will fully pass | Light fully/partially passed |
| Frosted glass | Translucent | Light will partially pass | Light partially passed |
Page No – 158
Table 11.2: Observation of shadows.
| Action | Observations regarding shadow |
| The screen is removed. | |
| The object is removed. | |
| The torch is switched off | |
| The object is moved closer to the screen, keeping the torch and the screen fixed. | |
| The object is moved closer to the torch, keeping the torch and the screen fixed. | |
| The object is tilted, keeping the torch and the screen fixed. | |
| The colour of the object is changed. |
Ans:
| Action | Observations regarding shadow |
| The screen is removed. | Shadow is not visible |
| The object is removed. | No shadow is formed |
| The torch is switched off | No shadow is formed |
| The object is moved closer to the screen, keeping the torch and the screen fixed. | Shadow becomes sharper and smaller |
| The object is moved closer to the torch, keeping the torch and the screen fixed. | Shadow becomes larger and blurries |
| The object is tilted, keeping the torch and the screen fixed. | Shape of the shadow changes |
| The colour of the object is changed. | No significant change in the shadow (shadow remain dark) |
| Let Us Enhance Our Learning |
1. Which of the following are luminous objects?
Mars, Moon, Pole Star, Sun, Venus, Mirror.
Ans: Pole Star and Sun are luminous objects.
2. Match the items in Column A with those in Column B.
| Column A | Column B |
| Pinhole camera | Blocks light completely |
| Opaque object | The dark region formed behind the object |
| Transparent object | Forms an inverted image |
| Shadow | Light passes almost completely through it |
Ans:
| Column A | Column B |
| Pinhole camera | Forms an inverted image |
| Opaque object | Blocks light completely |
| Transparent object | Light passes almost completely through it |
| Shadow | The dark region formed behind the object |
3. Sahil, Rekha, Patrick, and Qasima are trying to observe the candle flame through the pipe as shown in Fig. 11.16. Who can see the flame?
Ans: Patrick and Qasima can see the flame.Light travels in straight lines this is a basic property of light called rectilinear propagation.
4. Look at the images shown in Fig. 11.17 and select the correct image showing the shadow formation of the boy.
Ans: The image (b) correctly shows a small shadow directly below the boy when the Sun is overhead.
5. The shadow of a ball is formed on a wall by placing the ball in front of a fixed torch as shown in Fig. 11.18. In scenario (i) the ball is closer to the torch, while in scenario (ii) the ball is closer to the wall. Choose the most accurate representation of the shadows formed in both scenarios from the options provided (a and b).
Ans: We observe two different positions of the ball placed in front of a fixed torch to form shadows on a wall. In scenario (i), the ball is placed closer to the torch, while in scenario (ii), it is placed nearer to the wall. When the ball is closer to the torch, as in scenario (i), it blocks a larger portion of the light, creating a bigger and blurrier shadow. This corresponds to shadow image (a), which shows a large and fuzzy shadow. On the other hand, in scenario (ii), where the ball is closer to the wall, the shadow formed is smaller and sharper because the light has less space to spread around the object. This matches shadow image (b). Therefore, the correct match is that in scenario (i), the shadow formed is shown in image (a), and in scenario (ii), the shadow is shown in image (b).
6. Based on Fig. 11.18, match the position of the torch in Column A with the characteristics of the ball’s shadow in Column B.
| Column A | Column B |
| If the torch is close to the ball | The shadow would be smaller |
| If the torch is far away | The shadow would be larger |
| If the ball is removed from the set-up | Two shadows would appear on the screen |
| If two torches are present in the set-up on the left side of the ball | A bright spot would appear on the screen |
Ans:
| Column A | Column B |
| If the torch is close to the ball | The shadow would be larger |
| If the torch is far away | The shadow would be smaller |
| If the ball is removed from the set-up | A bright spot would appear on the screen |
| If two torches are present in the set-up on the left side of the ball | Two shadows would appear on the screen |
7. Suppose you view the tree shown in Fig. 11.19 through a pinhole camera. Sketch the outline of the image of the tree formed in the pinhole camera.
Ans: If you look at the tree in Fig. through a pinhole camera, you will see its image upside down. The top of the tree will appear at the bottom and the bottom part will be at the top. This happens because light travels in straight lines and crosses at the tiny hole, flipping the image. The image will also look smaller than the actual tree, but you will still be able to make out the shape clearly.
8. Write your name on a piece of paper and hold it in front of a plane mirror such that the paper is parallel to the mirror. Sketch the image. What difference do you notice? Explain the reason for the difference.
Ans: When you write your name on a piece of paper and hold it in front of a plane mirror with the paper parallel to the mirror, the image of the name appears reversed from left to right. This effect is called lateral inversion. For example,is the word “AMBULANCE” written in reverse on the front of ambulances. When drivers see it in their rear-view mirrors, the reversed letters appear the correct way, helping them quickly read the word and give way. This effect is due to the way a plane mirror reflects light, causing a sideways flip of the image.
9. Measure the length of your shadow at 9 AM, 12 PM, and 4 PM with the help of your friend. Write down your observations:
(i) At which of the given times is your shadow the shortest?
Ans: I noticed that it was longest in the morning at 9 AM, shortest at 12 PM, and then long again by 4 PM.
(ii) Why do you think this happens?
Ans: This happens because at 12 PM, the Sun is almost directly overhead,so it casts a very short shadow. But in the morning and afternoon, the Sun is lower, and the light falls at an angle, which makes the shadow look much longer.
10. On the basis of following statements, choose the correct option. Statement A: Image formed by a plane mirror is laterally inverted. Statement B: Images of alphabets T and O appear identical to themselves in a plane mirror.
(i) Both statements are true.
(ii) Both statements are false.
(iii) Statement A is true, but statement B is false.
(iv) Statement A is false, but statement B is true.
Ans: (i) Both statements are true.
11. Suppose you are given a tube of the shape shown in the Fig. 11.20 and two plane mirrors smaller than the diameter of the tube. Can this tube be used to make a periscope? If yes, mark where you will fi x the plane mirrors.
Ans: Yes, the tube shown in Fig. can be used to make a periscope.
To do this, you need to fix two plane mirrors inside the tube. Place one mirror at the top end of the tube at a 45-degree angle facing downwards, and the second mirror at the bottom end at a 45-degree angle facing upwards.These mirrors should be placed so they are parallel to each other. This setup allows light from an object at the top to reflect off the first mirror, travel down the tube, hit the second mirror, and finally reach your eyes at the bottom allowing you to see over or around obstacles.
12. We do not see the shadow on the ground of a bird flying high in the sky. However, the shadow is seen on the ground when the bird swoops near the ground. Think and explain why it is so.
Ans: When a bird is flying high in the sky, its shadow on the ground is not visible because the bird is too far away from the ground. The sunlight spreads over a large area, making the shadow very small and faint, so we don’t notice it.
But when the bird swoops down closer to the ground, it becomes much closer to the surface. At this point, the sunlight falls more directly on the bird, and its shadow becomes sharper and more visible on the ground because the bird is much closer to the surface, making the shadow easier to see.
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