## Topic outline

• ### Light

##### Q53. What happens when a ray of light falls perpendicularly on the surface of a plane mirror?

Ans. The angle of incidence for such a ray of light is zero. Since the angle of incidence is zero, so according to the law of reflection, the angle of reflection should also be zero. This means that the reflected ray will also travel back from the mirror along the normal.

##### Q54. How many images of the coin will be seen when two plane mirrors are set at right angles to each other and a coin is placed in-between these two plane mirrors?

Ans. The formula to calculate the no. of images of an object placed between 2 plane mirrors is (360°/x°)−1; where ‘x’ is the angle of inclination.

90° - No. of images formed = (360°/90°)-1= 4-1 =3

##### Q55. 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

##### Q56. What is the angle of incidence of a ray if the reflected ray is at an angle of 90° to the incident ray?

Ans. We know that the angle of incidence is equal to the angle of reflection.

According to the given condition

##### Q57. What is cataract? How can the vision of a person having cataract be restored?

Ans. Sometimes, particularly in old age, eyesight becomes foggy. It is due to the eye lens becoming cloudy. When it happens, persons are said to have cataract. There is a loss of vision, sometimes extremely severe. It is possible to treat this defect. The opaque lens is removed and a new artificial lens is inserted. Modern technology has made this procedure simpler and safer.

##### Q58. Suppose you are in a dark room. Can you see objects in the room? Can you see objects outside the room? Explain.

Ans. If we are in dark room, then it is not possible for us to see the objects in the room. However objects outside the room are visible to us. This is so, because eyes alone cannot see any object. It is only when light from an object enters our eyes that we see the object. The light may have been emitted by the object, or may have been reflected by it.

##### Q59. Draw a labeled sketch of the human eye.

Ans.

From NCERT

##### Q60. How many images of an object will be formed when two plane mirrors are set at an angle of 45°, 60°, 120° and 180°.

Ans. The formula to calculate the no. of images of an object placed between 2 plane mirrors is (360°/x°)−1; where ‘x’ is the angle of inclination.

45° - No. of images formed = (360°/45°)-1= 8-1 =7

60° - No. of images formed = (360°/60°)-1= 6-1 =5

120° - No. of images formed = (360°/120°)-1= 3-1 =2

180° - No. of images formed = (360°/180°)-1= 2-1 =1

##### Q61. How does eye adjust itself to deal with light of varying intensity?

Ans. The iris controls the amount of light entering into the eye by automatically adjusting the size of the pupil according to the intensity of the light that the eye receives. If the amount of light is high, the iris contracts the pupil and reduces the amount of light entering the eyes. If the amount of light is less, the iris expands the pupil so that more light can enter the eye and the things can be viewed clearly.

##### Q62. Explain why, an owl can see well in the night but not during the day whereas an eagle can see well during day but not in the night.

Ans. A night bird (owl) can see very well in the night but not during the day. On the other hand, day light birds (kite, eagle) can see well during the day but not in the night. The Owl has a large cornea and a large pupil to allow more light in its eye. Also, it has on its retina a large number of rods and only a few cones. The day birds on the other hand, have more cones and fewer rods.