JEE Main 2025 · 23 January · evening · Single correct · +4 / −1

A concave mirror of focal length f in air is dipped in a liquid of refractive index μ. Its focal length in the liquid will be:

  1. (a) μ f
  2. (b) f
  3. (c) f/μ
  4. (d) f/(μ-1)
Answer and solution

Answer (b) f

Solution

Physics sir · 3 steps

A mirror only reflects, and the law of reflection has no μ in it, so the liquid changes nothing.

  1. What sets f. For a spherical mirror, f = R/2. This comes purely from the geometry of reflection: the angle of incidence equals the angle of reflection.

    f = R/2

  2. Role of the medium. Reflection happens in the same medium on both sides of the ray, so the refractive index never enters the law of reflection. Dipping the mirror in the liquid does not change its radius of curvature R either.
  3. So. The focal length in the liquid stays f, which is option (b).

The idea to keep. A mirror's focal length depends only on its curvature, f = R/2. The surrounding medium changes a lens's focal length, never a mirror's.

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 2025.