NCERT Solutions for Class 7 Science Chapter 13 - Light

NCERT TEXTBOOK QUESTIONS SOLVED

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. We can see only those objects from which reflected rays enter our eyes. When we are in a dark room then we cannot see objects. We can see objects outside the room because out of the room the light is available and those rays of light enter our eyes after reflection.
Q. 2. Differentiate between regular and diffused reflection. Does diffused reflection mean the failure of the law of reflection?
 Ans. Difference between regular and diffused reflection:

Diffused reflection is not due to the failure of the laws of reflection. It is caused by the 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. 
(a) Polished wooden table
(b) Chalk powder
(c) Cardboard surface
(d) Marble floor with water spread over it.
(e) Mirror
(f) Piece of paper.
 Ans.  (a) A regular reflection takes place on a polished wooden table because its surface is smooth.
(b) A diffused reflection takes place on a chalk powder because its surface is uneven.
(c) Cardboard surface has minute irregularities on it, so it will have a irregular reflection.  
(d) Marble floor with water spread over it acts as a smooth surface for regular reflection as it will act like a plane surface.
(e) Mirror has a regular reflection. Its surface is also shiny and smooth.
(f) At a piece of a paper irregular or diffused reflection will take place because it has uneven surface. 

Q. 4. State the laws of reflection.
 Ans. There are two laws of reflection.
(i) Angle of incidence is always equal to the angle of reflection, i.e. ∠i = ∠r
(ii) The incident ray, the normal at the point of incidence and the reflected ray, all lie in the same plane. 

Q. 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.
Ans. For this experiment take a plane mirror and stand it on a plane sheet of paper with a block. Now draw an incidence line AB. Now see in the mirror and mark the points on the paper, where you find the line is travelling 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 see that incident ray, reflected ray and normal will be in the same plane, i.e. on the sheet of paper.

Q. 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.
 Ans. 
(a) 2
(b) left, left hand
(c) large
(d) lesser Choose the correct option in Questions 7-8:

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

Q. 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.
Ans. (b) Virtual, behind the mirror and of the same size as the object.
Q. 9. Describe the construction of a kaleidoscope.
Ans. To make a Kaleidoscope, take three rectangular mirror strips each about 15 cm long and 4 cm wide. Join them together to form a prism. Fix them in circular cardboard tube. Make sure that the tube is slightly longer than the mirror strips. Close one end of the tube by a cardboard disc having a hole in the centre. 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. On this glass plate, place several small pieces of coloured glass. Now close this end of the tube by a ground glass plate. Now, your kaleidoscope is ready.

Q . 10. Draw a labelled sketch of the human eye.
 Ans. 

Q. 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?
 Ans. Laser light is harmful for eye and can cause a permanent defect in the eye. She can lose her eyesight also.

Q . 12. Explain how you can take care of your eyes.
Ans. Do the following to care your eyes:
(i) Do not read in too little or too much light.
(ii) Wash your eyes with clean water at least three times a day.
(iii) Do not look at the sun or powerful light directly.
(iv) Always read at the normal distance for vision.
(v) Do not rub your eyes with dirty hand.

Q . 13. What is the angle of incidence of a ray if the reflected ray is at an angle of 90º to the incident ray?
Ans. The angle of incidence will be 45º.

Q . 14. How many images of a candle will be formed if it is placed between two parallel mirrors separated by 40 cm?
Ans. Infinite number of images will be formed.

Q . 15. Two mirrors meet at right angles. A ray of light is incident at an angle of 30º as shown in Fig. 16.10. Draw the reflected ray from the second mirror.

Ans.

Q . 16. Boojho stands at A just on the side of a plane mirror as shown in Fig. 16.11. Can he see himself in the mirror? Also can he see the image of objects situated at P, Q and R?

Ans. He cannot see his image himself. He can see only the image of P but not of Q and R.

Q . 17. (a) Find out the position of the image of an object situated at A in the plane mirror (Fig. 16.12).
 (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?

Ans. (a) 

(b) Yes, Paheli can see this image.
(c) Yes, Boojho can see this image.
(d) No change in the position of image A.