Question

A light ray is incident normally on a plane mirror.
(a)What is the angle of incidence?
(b)What is the path of the reflected ray?
A.(a)${180^ \circ }$ , (b)Same as incident ray
B.(a)${180^ \circ }$ , (b)Opposite to incident ray
C.(a)${0^ \circ }$ , (b)Opposite to incident ray
D.(a)${0^ \circ }$ , (b)Same as incident ray

Answer 1

Hint: We know that the angle of incidence is the angle made by the incident ray and the normal to the mirror surface at the point of incidence. In this case the incident ray and the normal coincides. The angle of reflection will be equal to the angle of incidence and direction will be opposite to incident direction.

Complete answer:
Given that a Ray of light is incident normally on a plane mirror. This means that the incident ray is making an angle ${90^ \circ }$ with the surface of the mirror.
(a)We need to find the angle of incidence.
Consider the ray diagram of normal incidence shown below.

The incident ray AB is falling on the mirror surface normally.
We know that the angle of incidence is the angle between the incident ray AB and the normal to the mirror surface drawn at the point of incidence.
In this case the normal to the mirror at the point of incidence and the incident ray AB coincides since both are making ${90^ \circ }$ angle with the mirror surface.
Which means there is no angle made between the normal and the incident ray. So, we can say that the angle of incidence is ${0^ \circ }$ .
(b) We know the two laws of reflection. They are,
(1) The incident ray, reflected ray and the normal lie in the same plane.
(2) The angle of incidence is the same as the angle of reflection.
The angle of reflection is the angle between the reflected ray BA and normal drawn at the point of incidence.
Since this angle is the same as the angle made between the incident ray AB and the normal. We can say that the angle of reflection is also zero.
The ray diagram will be as shown below.

The ray BA denotes the reflected ray. The direction of reflected ray BA is opposite to the direction of incident ray AB.
 So, the reflected ray BA will also coincide with the normal but the direction of the reflected ray is opposite to that of the incident direction.
So, the angle of incidence is ${0^ \circ }$ and direction of the path of the reflected ray is opposite to the incident ray.

Hence the correct answer is option C.

Note:
Always remember that the angle of incidence is the angle between the incident ray and the normal to the surface at the point of incidence. It is given that incident ray falls normally that is at an angle ${90^ \circ }$ don’t take this as the angle of incidence. angle of incidence is ${0^ \circ }$ since both the normal and incident ray coincides. Thus, the angle of reflection is also ${0^ \circ }$ .