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

JEE Main · 2025 · 23 Jan 2025 · Shift 2 · Geometrical Optics

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 :

(A) $\frac{f}{μ}$
(B) $\frac{f}{( μ - 1 )}$
(C) $μ f$
(D) $f$ ✓ Correct

Answer: (D)

Solution

The focal length of a spherical mirror is fixed purely by its geometry,

f = \frac{R}{2}

, since it works through reflection rather than refraction.

The law of reflection is unaffected by the medium surrounding the mirror, so immersing the mirror in a liquid of refractive index

μ

does not change its focal length.

The focal length in the liquid remains

f

JEE Main · 2025 · 23 Jan 2025 · Shift 2 · Single correct option

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