Chapter 12. LIGHT

Exercises

1. Suppose you are in a dark room. Can you see objects in the room? Can you see objects outside the room. Explain.

Answer : In a dark room, without any source of light, it is not possible to see objects in the room because there is no light available to reflect off the objects and reach our eyes. Similarly, without any light source outside the room, it is also not possible to see objects outside the room.

2. Differentiate between regular and diffused reflection. Does diffused reflection mean the failure of the laws of reflection?

Answer : The difference between regular and diffused reflection are :

Regular reflection : When all the parallel rays reflected from a rough or irregular surface are not parallel, the reflection is known as diffused or irregular reflection.

Diffused reflection : When all the parallel rays reflection from a smooth surface like that of a mirror is called regular reflection . Images are formed by regular reflection.

Diffused reflection does not indicate a failure of the laws of reflection. It simply means that the surface is not smooth enough to maintain regular reflection. The laws of reflection still hold true in diffused reflection, but the scattered rays make the reflection less organized and more diffused.

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
(c) Cardboard surface

(d) Marble floor with water spread over it
(e) Mirror

(f) Piece of paper

Answer : (a) Polished wooden table: Regular reflection will take place because the polished surface of the wooden table allows for the smooth and orderly reflection of light.

(b) Chalk powder: Diffused reflection will take place because the uneven and irregular surface of chalk powder scatters light in various directions, resulting in a diffused reflection.

(c) Cardboard surface: Diffused reflection will take place on a cardboard surface due to its rough and non-smooth texture.

(d) Marble floor with water spread over it: Regular reflection will take place as the smooth surface of the marble floor and the presence of water allow for the orderly reflection of light.

(e) Mirror: Regular reflection will take place on a mirror due to its highly polished and smooth surface, which reflects light in a well-defined and organized manner.

(f) Piece of paper: Diffused reflection will take place on a piece of paper because its surface is rough and irregular, causing light to scatter in different directions

4. State the laws of reflection.

Answer : (i) The angle of incidence is equal to the angle of reflection.

(ii) The incident ray, the reflected ray, and the normal to the surface at the point of incidence lie in the same plane.

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.

Answer : To demonstrate that the incident ray, the reflected ray, and the normal at the point of incidence lie in the same plane, we can perform the following activity:

(i) Take a plane mirror and place it on a flat surface.

(ii) Place a small object, such as a pencil or a coin, on the surface in front of the mirror.

(iii) Shine a light beam on the mirror, making sure it strikes the mirror surface and reflects off it.

(iv) Observe the incident ray, the reflected ray, and the normal at the point of incidence, and verify that they all 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 right hand in front of a plane mirror it will be seen in the mirror that your right ear is touched with ____________.
(c) The size of the pupil becomes ____________ when you see in dim light.
(d) Night birds have ____________ cones than rods in their eyes.
Choose the correct option in Questions 7 – 8

Answer : (a) A person 1 m in front of a plane mirror seems to be 2 m away from his image.

(b) If you touch your left ear with 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.

(c) The size of the pupil becomes larger when you see in dim light.

(d) Night birds have fewer rods than cones in their eyes.

7. Angle of incidence is equal to the angle of reflection.
(a) Always (b) Sometimes
(c) Under special conditions (d) Never

Answer : (a) always .

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.

Answer : (b) virtual, behind the mirror and of the same size as the object.

9. Describe the construction of a kaleidoscope.

Answer : A kaleidoscope is made up of a few simple parts. The main part is a long, narrow tube, usually made of cardboard or plastic. At one end of the tube, there is a small hole or opening for you to look through. At the other end, there is a round piece of glass or plastic called the object chamber.

Inside the object chamber, there are tiny objects like colorful beads, pieces of glass, or even small mirrors. These objects are placed in a way that they can move around freely when you shake or rotate the kaleidoscope.

When you look through the hole and move the kaleidoscope, the objects inside the object chamber create beautiful patterns. This happens because the light enters through the hole, reflects off the objects, and bounces around inside the kaleidoscope. The mirrors inside the tube reflect the light and create multiple reflections, forming the intricate patterns you see when you look through the kaleidoscope.

10. Draw a labelled sketch of the human eye.

Answer : Photo

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 advise?

Answer : The teacher advised Gurmit not to use a laser torch because laser beams are highly concentrated and can cause harm to the eyes. Laser light is coherent and intense, which means it can damage the retina of the eyes and cause vision problems. It is important to exercise caution while using lasers and follow safety guidelines to prevent any potential harm.

12. Explain how you can take care of your eyes.

Answer : To take care of your eyes, follow these practices:

(i) If advised, use suitable spectacles.

(ii)Too little or too much light is bad for eyes. Insufficient light causes eyestrain and headaches. Too much light, like that of the Sun, a powerful lamp or a laser torch can injure the retina.

(iii) Do not look at the Sun or a powerful light directly.

(iv) Never rub your eyes. If particles of dust go into your eyes, wash your eyes with clean water. If there is no improvement go to a doctor.

(v) Always read at the normal distance for vision. Do not read by bringing the book too close to your eyes or keeping it too far.

13. What is the angle of incidence of a ray if the reflected ray is at an angle of 90° to the incident ray?

Answer : The angle of incidence of a ray is 45° 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?

Answer :  If a candle is placed between two parallel plane mirrors separated by 40 cm, an infinite number of images of the candle will be formed. The images will be arranged in a regular pattern, with each image being a reflection of the previous image between the two mirrors.                         

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.

Answer : Photo

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 ?

Answer : Boojho can not see himself in the mirror.

Boojho can also see the image of objects situated at P and Q . but he can not see  R .

17. (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 move? 

Answer : (a) Photo

The position of the image of an object situated at A in the plane mirror would be located exactly behind the mirror at the same distance as object A.

(b) Paheli can see the image of object A .

(c) Boojho can see the image of object A.

(d) When Paheli moves from B to C, then the image of object A will not move .