﻿ 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?   : Kaysons Education

# 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?

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## Question

### Solution

Correct option is

1.55

In the first case we have               (âˆµ 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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