A lens of refractive index 1.6 has focal length 12 cm in air. Find its focal length when placed in water of refractive index 1.28.
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Question

A lens of refractive index 1.6 has focal length 12 cm in air. Find its focal length when placed in water of refractive index 1.28.

G1 Physics · Accepted Answer

For a thin lens in a medium, 1/f = (μ_lens/μ_medium - 1)(1/R₁ - 1/R₂). In air: 1/12 = (1.6-1)K = 0.6K, where K = (1/R₁ - 1/R₂). Thus, K = 1/(7.2 cm). In water: μ_rel = 1.6/1.28 = 1.25. So, 1/ f w f_w f w ​ = (1.25-1)K = 0.25/7.2. Therefore, f w f_w f w ​ = 7.2/0.25 = 28.8 cm = 288 mm. Correct option: B.