A narrow parallel beam of light is incident paraxially on a solid transparent sphere of radius ‘r’ kept in air. What should be the refractive index if the beam is to be focused at the farther surface of the sphere?

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1.5

1.3

answer is B.

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Detailed Solution

$\begin{array}{l} \frac{μ_{2}}{V} - \frac{μ_{1}}{u} = \frac{μ_{2} - μ_{1}}{R} \\ \frac{μ_{2}}{2 r} - \frac{1}{6} = \frac{μ_{2} - 1}{r} \\ \frac{μ_{2}}{2 r} = \frac{μ_{2} - 1}{r} \\ 2 μ_{2} - 2 = μ_{2} \\ μ_{2} = 2 \end{array}$

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