Class–VIII (CHAPTER-16) LIGHT Questions
1. Suppose you are in a dark room. Can you see objects in the room? Can you see objects outside the room? Explain.
2. Differentiate between regular and diffused reflection. Does diffused reflection mean the failure of the laws of reflection?
3. Mention against each of the following whether regular or diffused reflection will take place when a beam of light strikes. Justify your answer in each case.
(a) Polished wooden table (b) Chalk powder
(b) Cardboard surface (d) Marble floor with water spread over it
(e) irror (f) Piece of paper
4. State the laws of reflection.
5. Describe an activity to show that the incident ray, the reflected ray and the normal at the point of incidence lie in the same plane.
6. Fill in the blanks in the following.
(a) A person 1 m in front of a plane mirror seems to be m away from his image.
(b) If you touch your ear with your right hand in front of a plane mirror, it will be seen in the mirror that your right ear
is touched with your .
(c) The size of the pupil becomes when you see in dim light.
(d) Night birds have cones than rods in their eyes.
7. Angle of incidence is equal to the angle of reflection. (a) Always (b) Sometimes
(b) Under special conditions (d) Never
8. Image formed by a plane mirror is
(a) virtual, behind the mirror and enlarged.
(b) virtual, behind the mirror and of the same size as the object. (c) real at the surface of the mirror and enlarged.
(d) real, behind the mirror and of the same size as the object.
9. Describe the construction of a kaleidoscope.
10. Draw a labelled sketch of the human eye.
11. Gurmit wanted to perform Activity 16.8 using a laser torch. Her teacher advised her not to do so. Can you explain the basis of the teacher’s advice?
12. Explain how you can take care of your eyes.
13. What is the angle of incidence of a ray if the reflected ray is at an angle of 90° to the incident ray?
14. How many images of a candle will be formed if it is placed between two parallel plane mirrors separated by 40 cm?
15. Two mirrors meet at right angles. A ray of light is incident on one at an angle of 30° as shown in Fig. 16.19. Draw the reflected ray from the second mirror.
16. Boojho stands at A just on the side of a plane mirror as shown in
Fig. 16.20. Can he see himself in the mirror? Also can he see the image of objects situated at P, Q and R?
(a) Find out the position of the image of an object situated at A in the plane mirror (Fig. 16.21).
(b) Can Paheli at B see this image? (c) Can Boojho at C see this image?
(d) When Paheli moves from B to C, where does the image of A
Class–VIII (CHAPTER-16) LIGHT SCIENCE Answers
1. In a dark room a person cannot see any objects in the room, however objects outside the room would be visible.
The reason for this phenomenon is that an object becomes visible only when light reflected from the object reaches the eye. In a dark room, the objects cannot reflect any light as no light falls on the object whereas for the objects outside the room, the reflected light from the object reaches the eye, so it becomes visible.
2. Regular reflection means reflection taking place from a smooth and shiny surface. In case of regular reflection all the reflected rays are parallel to each other. Diffused or irregular reflection takes place from a rough or irregular surface. In this case, the reflected rays are not parallel to each other.
Diffused reflection does not mean failure of the laws of reflection as each and every follows the laws of reflection but due to irregularities in the surface, the reflected rays moves in different directions.
3. (a) Polished wooden table → Regular reflection
A polished surface means the surface is very smooth, thus regular reflection takes place.
(b) Chalk powder → Diffused reflection
If we observe chalk powders, we would find out that the surface of chalk powder spread is irregular. Thus, diffused reflection takes place.
(c) Cardboard surface → Diffused reflection
The surface of cardboard which reflects light is not smooth, thus diffused reflection will take place from a cardboard surface.
(d) Marble floor with water spread over it → Regular reflection The water spread over marble makes the surface very smooth. Hence, regular reflection takes place.
(e) Mirror → Regular reflection
The surface of mirror is very smooth which ensures regular reflection.
(f) Piece of paper → Diffused reflection
A piece of paper may look smooth, but it has many irregularities on its surface. Thus, diffused reflection takes place.
4. Laws of reflection can be stated as follows
(i) The angle of reflection and angle of incidence are always equal to each other.
(ii) The incident ray, the normal at the point of incidence and the reflected ray all lie in the same plane.
5. Take a plane mirror and stand it on a plane sheet of paper with a block. Now draw an incidence line AB. Now look in the mirror and mark the points on the paper, where you feel the line is traveling after getting reflected from the mirror. Remove the mirror and draw a perpendicular on the mirror line. Join the points to make the reflected ray. You will observe that incident ray, reflected ray
and normal will be in the same plane, i.e. on the sheet of paper.
(a) A person 1 m in front of a plane mirror seems to be 2 m away from his image.
(Object distance and image distance are the same from a plane mirror. The image of a person 1 m in front of a mirror is 1 m back to the mirror. Hence, the image is 1 + 1 = 2 m away from the person.)
(b) If you touch your left ear with your right hand in front of a plane mirror, it will be seen in the mirror that your right ear is touched with your left hand.
(This is because of lateral inversion of images formed in a plane mirror.)
(c) The size of the pupil becomes large when you see in dim light.
(In dim light, the amount of light entering the eye is very little. To increase the amount of light, the pupil expands.)
(d) Night birds have less cones than rods in their eyes.
(Night birds can see in the night, but not in the day. They have on their retina a large number of rod cells and only a few cones.)
7. (a) Always.
The first law of reflection states that the angle of incidence and angle of reflection are always equal to each other.
8. (b) virtual, behind the mirror and of the same size as the object.
The image formed by a plane mirror is formed behind the mirror. This image cannot be formed on a screen, thus it produces a virtual image which is of the same size as that of the object.
9. Kaleidoscope is made up of three rectangular mirror strips each about 15 cm long and 4 cm wide (lxb). These are joined together to form a prism. The prism is fixed in the circular cardboard tube. Make sure that the tube is slightly longer than the mirror strips. One end of the tube is closed by a cardboard disc having a hole in the center. To make the disc durable, paste a piece of transparent plastic sheet under the cardboard disc. At the other end, touching the mirrors, fix a circular plane glass plate.
11. A Laser light is harmful for eye and can cause a permanent defect in the eye as its intensity is very high. It can cause severe damage to the retina which could lead to blindness. Hence, it is advisable not to look at a laser beam directly.
12. To protect our eyes, the following precautions should be taken: (i) Avoid reading in dim and very bright light.
(ii) Wash your eyes with cold water regularly
(iii) Do not look at the sun or powerful light directly.
(iv) Always maintain the normal distance for vision while reading. (v) Do not rub your eyes with dirty hand.
13. Angle of incidence is equal to the angle of reflection. Thus, if the angle between the incident and the reflected ray is 90 degrees, the angle of incidence is equal to 90/2 = 45 degrees.
14. Infinite no of images will be formed due to multiple reflection between two parallel plane mirrors.
15. The first law of reflection is used to obtain the path of reflected light.
It can be observed that the given ray of light will reflect from the second mirror at an angle 60°.
16. A cannot see his image as the length of the mirror is too short on his side. He can see the image of the objects at P and Q but cannot see the object at R as can be inferred from the ray diagram.
17. (a) The image of A is formed at behind the mirror at an equal distance of A from the mirror.
(b) Yes, Paheli can see the image. (c) Yes, Boojho can see the image.
(d) The image of A is unaffected by the move