The radii of curvatures of convex surface, concave surface of a concavo-convex lens are in the ratio 1 : 2. The refractive index of the material of lens is 5/4 and its focal length is 40 cm. This lens is cut into a plano-convex lens and plano-concave lens. If the plano convex lens is immersed in water then its focal length is

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+ 20 cm

-20 cm

+ 80 cm

-80 cm

answer is D.

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Detailed Solution

1f=μ−11R−12R140=54−112RR=5 cm1f2=μμw−11R1f2=5/44/3−115=-180f2=-80 cm

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