Ncert Exercises & Solutions

The radius of curvature of the curved surface of a plano-convex lens is 20 cm. If the refractive index of the material of the lens be 1.5, it will:

1. act as a convex lens only for the objects that lie on its curved side.
2. act as a concave lens for the objects that lie on its curved side.
3. act as a convex lens irrespective of the side on which the object lies.
4. act as a concave lens irrespective of the side on which the object lies.

(3) Hint: The nature of the prism depends on its refractive index and its radius of curvature.

Step 1: Use the lens maker's formula.

Here, R= 20cm, $μ$ = 1.5, on substituting the values in $f = \frac{R}{μ - 1} = \frac{20}{15 - 1} = 40 cm$ of converging nature as f>0. Therefore, the lens act as a convex lens irrespective of the side on which the object lies.

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