Question

If the angle of incidence on a plane mirror is ${45^ \circ }$. What will be its angle of reflection?
A) ${35^ \circ }$
B) ${45^ \circ }$
C) ${135^ \circ }$
D) ${145^ \circ }$

Answer 1

Hint: Angle of incidence is the angle that the incident ray makes with the normal to the mirror. Angle of reflection is the angle that reflected ray makes with the normal to the mirror. The relation between the angle of incidence and angle reflection is given by Snell’s law but it is in cases of refraction.

Complete step by step solution:
Whenever light rays strike on a surface; it strikes at some angle. This angle is measured from the normal to the surface. Normal to the surface is a line which is perpendicular to the surface. Now all the angles are measured with respect to this line. Angle of reflection too, is measured with respect to normal.
The law of reflection states that when a ray of light reflects off a surface, the angle of incidence and the angle of reflection are equal. As, the angle of incidence and angle of reflection is the same for a plane mirror. As the angle of incidence is ${45^ \circ }$ hence, the angle of reflection will also be ${45^ \circ }$.

Therefore, Option B is the correct option.

Note: Remember laws of reflection, in order to answer this question. Also, it must be noted that the incident ray and reflected rays are on the same side of a planar mirror.
In this case the reflected ray is perpendicular to the incident ray. But this is not true for any angle other than ${45^ \circ }$ . Also remember, when an incident ray strikes perpendicularly to the surface than the reflected ray traces the same path. The angle of incidence and the angle of reflection in that case would be ${0^ \circ }$.