A ray of light is incident on a plane mirror at an angle 30° with the normal at the point of incidence. The ray will be deviated from its incidence direction by what angle ?

According to the laws of reflection, the angle of incidence (i) is equal to the angle of reflection (r) [1]. In this case, the ray is incident at 30° with the normal, so the angle of reflection is also 30° [t1][t2]. The angle of deviation (δ) is defined as the angle between the original direction of the incident ray and the final direction of the reflected ray. Geometrically, if the mirror were absent, the ray would continue in a straight line. The reflection causes the ray to turn away from this path. The formula for the angle of deviation for a single reflection from a plane mirror is δ = 180° - (i + r) or δ = 180° - 2i [t1][t2]. Substituting the given value, δ = 180° - 2(30°) = 180° - 60° = 120°. Therefore, the ray is deviated by 120° from its original incidence direction.

Sources

1. [1] Science , class X (NCERT 2025 ed.) > Chapter 9: Light – Reflection and Refraction > 9.1 REFLECTION OF LIGHT > p. 135