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) [c2][t1]. In this case, the ray is incident at 30° with the normal, so i = 30° and r = 30° [c1][t1]. 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 travel in a straight line (180°). The deviation is calculated using the formula δ = 180° – (i + r) or δ = 180° – 2i [t2]. Substituting the given value, δ = 180° – 2(30°) = 180° – 60° = 120° [t2][t7]. While some sources confuse deviation with the angle between rays (i+r=60°), the standard physical definition of deviation from the original path is 120°.

Sources

1. [1] Science ,Class VIII . NCERT(Revised ed 2025) > Chapter 10: Light: Mirrors and Lenses > Activity 10.4: Let us experiment > p. 158
2. [2] Science , class X (NCERT 2025 ed.) > Chapter 9: Light – Reflection and Refraction > 9.1 REFLECTION OF LIGHT > p. 135