JEE Main 2023 · 11 April · evening · Single correct · +4 / −1

When one light ray is reflected from a plane mirror with 30° angle of reflection, the angle of deviation of the ray after reflection is :

  1. (a) 140°
  2. (b) 130°
  3. (c) 120°
  4. (d) 110°
Answer and solution

Answer (c) 120°

Solution

Physics sir · 3 steps

Deviation is how far the ray turns from its original path, so measure it from the straight-through line.

  1. Angle of incidence. By the law of reflection, i = r. So the angle of incidence is also 30°, measured from the normal.
  2. Find the turn. With no mirror, the ray would go straight on. The incident and reflected rays are each 30° from the normal, so the angle between them is 2i = 60°. The ray turns through what is left of the straight angle.

    δ = 180° - 2i = 180° - 2 × 30° = 120°

  3. So. The ray is deviated by 120°, which is option (c).

The idea to keep. For a plane mirror, deviation is 180° - 2i. It is largest (180°) at normal incidence and shrinks as the ray strikes more and more grazingly.

Answer from the NTA final answer key. Solution by PrepWiser.

Study Ray Optics

Asked about 200 times in JEE Main since 2020. This question is from 2023.