Two Plano concave lenses of glass of refractive index 1.5 have radii of curvature 20 cm and 30 cm respectively. They are placed in contact with the curved surface towards each other and the space between them is filled with a liquid of refractive index 5/2. The focal length of the combination is (in cm)

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108

answer is D.

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Detailed Solution

From lens maker’s formula, effective focal length is 1F=(μ1−μg)(1R1+1R2) =(52−32)(120+130) 1F⇒112⇒F=12cm

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