Question

A convex lens of power P is immersed in water. How will its power change ?

• A. Increase
• B. Remain unchanged
• C. Decrease
• D. Increase for red colour and decrease for blue colour.

Answer 1

Answer: (C)

Decrease

Answer 2

$p=\frac{1}{f}=(n_2-n_1)\left[\frac{1}{R_1}-\frac{1}{R_2}\right]$ Here $n _1$ is the refractive index of the medium in which lens is placed. Since $n_1$, for air is less than that for water. Hence P is less in water.