Question

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

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

Answer 1

The correct option is C act as a convex lens irrespective of the side on which the object lies.

Here, $\mu =1.5$

If object lies on plane side; $R_1=\infty ,R_2=-20cm$

$\frac{1}{f}=(\mu -1)(\frac{1}{R_1}-\frac{1}{R_2})$

$=(1.5-1)(\frac{1}{\infty }+\frac{1}{20})=\frac{1}{40}$

f = + 40 cm. The lens behaves as convex lens. If object lies on its curved side. Then $R_1=20cm,R_2=\infty$

$\frac{1}{f^{\prime }}=(\mu -1)(\frac{1}{R_1}-\frac{1}{R_2})$

$=(1.5-1)(\frac{1}{20}+\frac{1}{\infty })=\frac{1}{40}$

f' = 40 cm. The lens behaves as convex lens.