A plano-convex lens acts like a concave mirror of 28 cm focal length when its plane surface is silvered and like a concave mirror of 10 cm focal length when its curved surface is silvered. What is the refractive index of the material of the lens?  


Correct option is



In the first case we have               (∵ fm = ∞) 


In the second case we have


Where fm is the focal length of the curved silvered surface. Hence fm = R/2 where R is the radius of curvature of the curved surface.

Subtracting (i) and (ii) we get



From (i) we have f1 = 28 × 2 = 56 cm. Now   






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