A thin convex lens of refractive index 1.5 has 20 c m focal length in air. If the lens is completely immersed in a liquid of refractive index 1.6 , its focal length will be

Question

A thin convex lens of refractive index 1.5 has 20 c m focal length in air. If the lens is completely immersed in a liquid of refractive index 1.6 , its focal length will be, - 160 c m, - 100 c m, + 160 c m, + 100 c m

MARKS App · Accepted Answer

From lens formula 1 f = μ g a - 1 1 R 1 - 1 R 2 = 1.5 - 1 1 R 1 - 1 R 2 .....(i) Also, μ g l = μ g μ l = 1.5 1.6 ∴ 1 f ′ = μ g - 1 l 1 R 1 - 1 R 2 1 f ′ = 15 16 - 1 1 R 1 - 1 R 2 ...(ii) Dividing Equation (i) by Equation (ii), we get f f ′ = 15 16 - 1 1.5 - 1 = - 1 8 f ' = − 8 f = − 160 c m

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