Q .     A plano-convex lens of thickness `4` cm, when placed on a horizontal table, with the curved surface in contact with it, the apparent depth of the bottom most point of the lens is found to be `3` cm. If the lens is inverted such that the plane face is in contact with the table, the apparent depth of the centre of the plane face is found to be `25/8` cm. Find the focal length of the lens. Assume thickness to be negligible while finding its focal length.

JEE 1984
A

`25` cm

B

`30` cm

C

`15` cm

D

`10` cm

HINT

`d_text(app) = d_text(actual)/μ` Apply `μ_2/v − μ_1/u = (μ_2−μ_1)/R`

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