A planoconvex lens becomes an optical system of $28$ $cm$ focal length when its plane surface is silvered and illuminated from left to right as shown in Fig-A. If the same lens is instead silvered on the curved surface and illuminated from other side as in Fig-B, it acts like an optical system of focal length $10$ $cm.$ The refractive index of the material of lens is :

Question

A planoconvex lens becomes an optical system of $28$ $cm$ focal length when its plane surface is silvered and illuminated from left to right as shown in Fig-A.

If the same lens is instead silvered on the curved surface and illuminated from other side as in Fig-B, it acts like an optical system of focal length $10$ $cm.$ The refractive index of the material of lens is :

$1.50$

$1.55$

$1.75$

$1.51$

Solution

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