The two surfaces of a biconvex lens has same radii of curvatures. This lens is made of glass of refractive index 1.5 and has a focal length 10 cm in air. The lens is cut into two equal halves along a plane perpendicular to its principal axis to yield two plano–convex lenses. The two pieces are glued such that the convex surfaces touch each other. If this combination lens is immersed in water of refractive index 4/3 its focal length (in cm) is

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answer is D.

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Detailed Solution

Focal length of each part=2f

$\Rightarrow f_{1} = f_{2} = 20 cm;$

for the combination,

focal length = f' = 10 cm
When immersed in water
$\begin{array}{l} f_{L} = f' \frac{(μ_{g} - 1) μ_{w}}{(μ_{g} - μ_{w})} \\ f_{L} = 40 cm \end{array}$

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