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A narrow pencil of parallel light is inc...

A narrow pencil of parallel light is incident normally on a solid transparent sphere of radius r. What should be the refractive index if the pencil is to be focussed (a) at the surface of the sphere, (b) at the centre of the sphere.

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The correct Answer is:
(a) 2, (b) not possible, it will focus close to the centre if the refractive index is large
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