Ray Optics: Lens — Focal Length in Different Media MCQ

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Lens — Focal Length in Different MediaHardQuestion 20243

Question

A convex glass lens (n = 1.5) has focal length 10 cm in air. When immersed in water (n = 4/3), its focal length becomes:

A
40 cm
Correct
B
30 cm
C
20 cm
D
10 cm

Correct answer

40 cm

Explanation

1/f ∝ (n_lens/n_medium − 1). In air: 1/10 = (1.5−1)(1/R₁−1/R₂) = 0.5K. In water: 1/f_w = (1.5/(4/3)−1)K = (1.125−1)K = 0.125K = (0.125/0.5)×(1/10) = (1/4)×(1/10) → f_w = 40 cm.

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