16 LIGHT 18 TEXTBOOK QUESTIONS AND THEIR ANSWERS Q.1. Suppose you are in a dark room. Can you see objects in the room? Can you see objects outside the room? Explain. Ans. When we are in a dark room, we cannot see the objects in the room because there is no light in the room and objects are visible only when they reflect light from their surface. We can see objects outside the room because objects outside the room are in light. Light energy becomes off from the surface of object and reaches our eyes and helps us to see the object. Q.2. Differentiate between regular and diffused reflection. Does diffused reflection means the failure of laws of reflection? Ans. Regular reflection : When all the parallel rays reflected from a plane surface are parallel to each other, it is called regular reflection as shown in figure. 1
Diffused reflection : When all the parallel rays reflected from a surface are not parallel, to each other the reflection is known as diffused reflection or irregular reflection as shown in figure. The diffused reflection is not due to the failure of laws of reflection but caused by irregularities in the reflecting surface. Q.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. Polished wooden table, chalk powder, cardboard surface, marble stone with water spread over it, mirror and a piece of paper. Ans. (a) Polished wooden table It will have regular reflection as its surface is smooth. (b) Chalk powder Chalk powder will have diffused reflection as chalk particles are not smooth. 2
(c) Cardboard surface It will have diffused reflection as the surface of the cardboard has minute irregularities. (d) Marble stone with water spread over it It will have regular reflection if the water is still, and diffused reflection if waves are formed on the water surface. (e) Mirror It will have regular reflection because of its smooth surface. (f) A piece of paper It will have diffused reflection due to minute irregularities on its surface. Q.4. State the laws of reflection. Ans. Laws of reflection states that (a) Incident ray, reflected ray and normal drawn at the point of incidence to the reflecting surface always lie in the same plane. (b) Angle of incidence is always equal to the angle of reflection. Q.5. Fill in the blanks in the following : (a) A person 1 m in front of a plane mirror seems to be m from his image. 3
(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. Ans. (a) 2m, (b) left, left, (c) larger, (d) lesser Q.6. Angle of incidence is equal to the angle of reflection (a) Always (b) Sometimes (c) Under special conditions (d) Never Ans. (a) Q.7. 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. 4
Ans. (b) Q.8. Draw a labelled sketch of human eye. Q.9. Gurmit wanted to using a laser torch. Her teacher advised her not to do so. Can you explain the basis of the teachers advise? Ans. Teacher advised Gurmit not to put laser light using laser torch in her friend s eyes because laser light is harmful for the eyes and can even cause permanent damage when thrown directly into the eyes. Q.10. Explain how you can take care of your eyes. Ans. The following precautions should be taken in order to take care of your eyes. (a) We should always wash at least three times daily with clean cold water. 5
(b) We should not touch our eyes with dirty hands. (c) We should not look at the sun with naked eyes. (d) We should read books and watch television from a suitable distance. (e) No pointed objects should be brought near the eyes. Q.11. What is the angle of incidence of a ray if the reflected ray is at an angle of 90 to the incident ray? Ans. If the reflected ray is at an angle of 90 to the incident ray, the angle of incidence will be of 45. Q.12. How many images of a candle will be formed if it is placed between two parallel plane mirrors separated by 40 cm? Ans. If a candle is placed between two parallel plane mirrors separated by 40cm, then infinite images will be formed. 6
Q.13. Can you see an object in the dark? Ans. No, we cannot see an object in the dark. Q.14. How is seeing made possible? Ans. When light rays get reflected from the object to the eyes, seeing is made possible. Q.15. How can you see the hairs at the back of your head? Ans. We can see the hairs at the back of our head with the help of two plane mirrors. Q.16. What happens if two plane mirrors in combination is used? Ans. If two plane mirrors are used in combination then multiple images are formed. Q.17. Can you explain how reflection from two mirrors enables you to see objects which are not visible directly? Ans. Reflection from two plane mirrors placed at an angle enable us to see objects which are not visible directly. For example, periscope consisting two plane mirrors, is used for seeing such objects which are not visible directly. 7