What is the angle between the incident and reflected rays when a ray of light is incident normally on a plane mirror?
A. ${90^ \circ }$
B. ${45^ \circ }$
C. ${180^ \circ }$
D. ${0^ \circ }$

Hint: To solve this question, it is important to understand the principle of reflection. Reflection is defined as the change in direction of the ray of light in the same medium after bouncing off a flat, shining surface.

Complete step-by-step answer:
Consider a ray of light undergoing reflection as follows:

There are two laws governing the reflection known as, the laws of reflection: They are:
i)The incident ray, normal and the reflected ray lie on the same plane and are in the same medium.
ii)The angle made by the incident ray is equal to the angle made by the reflected ray.
Here, it says, that
Angle of incidence, $i$= Angle of reflection, $r$
In the problem, it is given that the ray of light is normally incident on the plane mirror. This means that the angle of incidence, $i = {0^ \circ }$
As per the law of reflection, $i = r$
So, therefore, the angle of reflection, $r = i = {0^ \circ }$

Hence, the correct option is Option D.

Note: In this question, the light is considered to be a ray. In the olden days during Newton’s times, the light was considered to be a ray. However, the experiments by scientists like Huygen and Maxwell, proved that the light is not a ray, but a wave. So, it does not mean that the concept of the reflection is refuted.
After the concept of Huygens wavefronts were discovered, the ray of light simply meant the perpendicular direction of the wavefront. So, the reflection of light meant the reflection of the wavefront of light.