Question

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

• A. act as a concave lens irrespective of side on which the object lies.
• B. act as convex lens only for the objects that lie on its curved side.
• C. act as a concave lens for the objects that lie on its curved side.
• D. act as a convex lens irrespective of the side on which the object lies.

Answer 1

Answer: (D)

Given, $R=20cm,\mu =1.5=\frac{3}{2}$

Then, $R_1=0,R_2=-20cm$

$\frac{1}{f}=(\mu -1)[\frac{1}{R_1}+\frac{1}{R_2}]=(1.5-1)[0+\frac{1}{20}]\Rightarrow f=40cm$

Thus it behaves as a convex lens.