Which of the following are luminous objects?
Mars, Moon, Pole Star, Sun, Venus, Mirror
Answer
Sun and Pole Star are luminous objects.
Luminous objects are objects that emit their own light. The Sun and the Pole Star (a star) emit their own light, so they are luminous. Mars, the Moon, Venus, and the Mirror do not emit their own light; they are seen only because they reflect the light falling on them, so they are non-luminous.
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 |
Answer
| 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 |
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?
Answer
Rekha can see the candle flame.
Light travels in a straight line. The flame can be seen only by the person whose part of the pipe provides a straight path between the eye and the candle. Only Rekha's pipe is aligned in a straight line with the candle, so light from the flame can reach her eye. The paths to the others are bent, so light cannot reach their eyes through the pipe.
Look at the images shown in Fig. 11.17 and select the correct image showing the shadow formation of the boy.
Answer
Image (d) shows the correct shadow formation of the boy.
Reason — A shadow is always formed on the side opposite to the source of light and is joined to the feet of the object. In image (d), the shadow falls on the side away from the Sun and remains attached to the boy's feet, which is the correct way a shadow forms.
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).
Answer
In scenario (i), the shadow is the larger one shown in option (b).
In scenario (ii), the shadow is the smaller one shown in option (a).
Reason — The size of a shadow depends on the position of the object between the light source and the wall. When the ball is closer to the torch, its shadow on the wall is larger. When the ball is closer to the wall, its shadow is smaller.
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 |
Answer
| 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 |
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.
Answer
A pinhole camera forms an inverted image. So, the image of the tree seen in the pinhole camera will be upside down, with the top of the tree pointing downward and the roots pointing upward.
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.
Answer
In the mirror, the name appears reversed from left to right, that is, the letters look back-to-front.
Difference noticed — The image in the mirror is laterally inverted. The left side of the writing appears on the right and the right side appears on the left.
Reason — This happens because of lateral inversion. In an image formed by a plane mirror, the left of the object appears as the right and the right of the object appears as the left.
Measure the length of your shadow at 9 AM, 12 PM, and 4 PM with the help of your friend. Write down your observations:
- At which of the given times is your shadow the shortest?
- Why do you think this happens?
Answer
- The shadow is the shortest at 12 PM (noon).
- At noon, the Sun is almost directly overhead, so its light falls on us in a nearly straight downward direction. This forms the shortest shadow. At 9 AM and 4 PM, the Sun is lower in the sky and its light falls at a slanting angle, so the shadows formed are longer.
On the basis of the 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.
- Both statements are true
- Both statements are false
- Statement A is true, but statement B is false
- Statement A is false, but statement B is true
Answer
Both statements are true
Reason — Statement A is true because a plane mirror always forms a laterally inverted image, in which the left appears as the right and the right appears as the left. Statement B is also true because the letters T and O are symmetrical about their vertical line. So, even after lateral inversion, their images look exactly the same as the original letters.
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 fix the plane mirrors.
Answer
Yes, this tube can be used to make a periscope.
The two plane mirrors should be fixed at the two bends of the tube. Each mirror should be inclined at about 45° to the path of light, with the reflecting surfaces facing each other. Light entering through the upper opening strikes the first mirror, is reflected towards the second mirror, and then reaches the eye after a second reflection.
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.
Answer
When the bird flies high in the sky, it is very far from the ground, which acts as the screen. At such a large distance, the shadow becomes very faint and spread out, so it is not seen on the ground.
When the bird swoops near the ground, it is much closer to the screen. Therefore, a sharper and more distinct shadow is formed on the ground, which can be clearly seen.