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 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

Answer: (C)

Act as a convex lens irrespective of the side on which the object lies.

Answer 2

: Here, $\mu = 1.5$

If object lies on plane side; $\mathrm { R } _ { 1 } = \infty , \mathrm { R } _ { 2 } = - 20 \mathrm {~cm}$

$\frac { 1 } { \mathrm { f } } = ( \mu - 1 ) \left( \frac { 1 } { \mathrm { R } _ { 1 } } - \frac { 1 } { \mathrm { R } _ { 2 } } \right)$

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

F = + 40 cm. The lens begaves as convex lens.

If object lies on its curved side. Then

$\frac { 1 } { \mathrm { f } } = ( \mu - 1 ) \left( \frac { 1 } { \mathrm { R } _ { 1 } } - \frac { 1 } { \mathrm { R } _ { 2 } } \right)$

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

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