A plano-convex lens having radius of curvature of first surface 2 cm exhibits focal length of f 1 in air. Another plano-convex lens with first surface radius of curvature 3 cm has focal length of f 2 when it is immersed in a liquid of refractive index 1.2. If both the lenses are made of same glass of refractive index 1.5, the ratio of f 1 and f 2 will be :-

Physics · Geometrical Optics

JEE Main 2025 — 24 January, Morning Shift — Question 61

A plano-convex lens having radius of curvature of first surface 2 cm exhibits focal length of $f_{1}$ in air. Another plano-convex lens with first surface radius of curvature 3 cm has focal length of $\mathrm{f}_{2}$ when it is immersed in a liquid of refractive index 1.2. If both the lenses are made of same glass of refractive index 1.5, the ratio of $\mathrm{f}_{1}$ and $\mathrm{f}_{2}$ will be :-

Option A:
$3: 5$
Option B:
$1: 3$
Correct
Option C:
$1: 2$
Option D:
$2: 3$

Answer: B

Step-by-step solution

$\frac{1}{\mathrm{f}_{1}}=(1.5-1)\left[\frac{1}{2}-0\right] \Rightarrow \mathrm{f}_{1}=4 \mathrm{~cm}$

$\frac{1}{\mathrm{f}_{2}}=\left(\frac{1.5}{1.2}-1\right)\left(\frac{1}{3}-0\right)$

$\frac{1}{\mathrm{f}_{2}}=\frac{0.3}{1.2} \times \frac{1}{3}$

$\mathrm{f}_{2}=12$

$\mathrm{f}_{1}: \mathrm{f}_{2}=4: 12=1: 3$

Answer key and solution verified before publishing.

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Exam: JEE Main 2025
Paper: 24 January, Morning Shift
Subject: Physics
Chapter: Geometrical Optics
Topic: Lenses and Their Combinations, Silvering of Lens

Answer: B

Step-by-step solution

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