Consider the path of a ray of light passing through a rectangular glass slab for different angles of incidence.

(I) Which one is greater: angle of incidence or angle of refraction?

(ii) What happens to the emergent angle on increasing the incident angle at the air-glass interface?

(iii) State the conditions when no refraction occurs.

OR

Sunita takes a mirror which is depressed at the centre and mounts it on a mirror stand. An erect and enlarged image of her face is formed. She places the mirror on a stand along a meter scale at 15 cm mark. In front of this mirror, she mounts a white screen and moves it back and forth along the meter scale till a sharp, well-defined inverted image of a distant tree is formed on the screen at 35 cm mark.

(i) Name the mirror and find its focal length.

(ii) Why does Sunita get a sharp image of the distant building at 35 cm mark?

Angle of incidence is the angle formed between the incident ray and the normal at the air-glass interface.

The angle of refraction is the angle formed between the refracted ray and the normal.

i) When the ray of light travels from rarer medium to denser medium, the speed of the light ray slows down, and hence the ray gets bent towards the normal.

Since the ray of light bends towards the normal, the angle of refraction is less than the incident angle.

So in this case the angle of incidence is greater than the angle of refraction.

ii) For any interface, the angle of emergence is always equal to the incident angle, and hence when we try to increase the angle of incidence, the angle of emergence also increases.

iii) Refraction basically occurs due to the change in the speed of light as the light ray travels from one medium to another of different refractive indices.

So the two where no refraction will take place are as follows:

1. When the incident ray is perpendicular to the medium interface.

2. When the ray of light travels from one medium to another of same refractive indices.

OR

(i) The mirror is a concave mirror as the concave mirrors are only depressed at the centre and the image formed is erect and enlarged which is normally formed by concave mirrors only.

The focal length of the mirror can be determined as follows:

The distance at which the image is formed = 35 cm

The distance at which the mirror is placed = 15 cm.

Focal Length, F = 35 – 15

= 20 cm.

(ii) The main reason for the sharp image of the distant building is that the incident rays from the object are parallel to each other and after being reflected from the concave mirror, the rays converge at the focus on the principal axis.