A biconvex lens of refractive index 1.5 has a focal length of 20 cm in air. Its focal length when immersed in a liquid of refractive index 1.6 will be:
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A biconvex lens of refractive index 1.5 has a focal length of 20 cm in air. Its focal length when immersed in a liquid of refractive index 1.6 will be:

G1 Physics · Accepted Answer

Given, μ(lens) = 1.5 μ(medium) = 1.6 f(air) = 20 cm Using lens maker relation in a medium: f(medium)/f(air) = [(μ(lens) - 1)μ(medium)]/(μ(lens) - μ(medium)) Therefore, f(medium)/20 = [(1.5 - 1) × 1.6]/(1.5 - 1.6) = 0.8/(-0.1) = -8 Hence, f(medium) = -160 cm