When plane surface of planoconvex lens is silvered its focal length comes out to be 28 cm. When convex surface is silvered focal length comes out to be 10 cm. The refractive index of lens is

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14/9

3.8

1.9

2.0

answer is A.

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Detailed Solution

$F_{1} = \frac{R}{2 (μ - 1)}$ $\Rightarrow 28 = \frac{R}{2 (μ - 1)}$

$F_{2} = \frac{R}{2 μ}$ $\Rightarrow 10 = \frac{R}{2 μ}$

$\frac{28}{10} = \frac{1}{(μ - 1)} \times \frac{μ}{1}$

$\Rightarrow 28 μ - 28 = 10 μ$

$\Rightarrow 18 μ = 28 \Rightarrow μ = \frac{28}{18} = \frac{14}{9}$

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