Question

A convex glass lens is immersed in water. Compared to the power in air, its power in water will :

• A. increase
• B. decrease
• C. not change
• D. decrease for red light and increase for violet light

Answer 1

Answer: (B)

decrease

Power of a lens $=\frac{1}{f}$

Where $f$ is the focal length of the lens.

So, by lens maker formula

$\frac{1}{f}=(\frac{{\mu }_2}{{\mu }_1}-1)(\frac{1}{R_1}-\frac{1}{R_2})$

As lens is dipped in water, $\mu$ of the medium is increased from 1 to $\frac{4}{3}$ (i.e. refractive index of water), so, $\frac{{\mu }_2}{{\mu }_1}$ term decreases, so, $(\frac{{\mu }_2}{{\mu }_1}-1)$ term decrease which leads to decrease of $\frac{1}{f}.$ so, power $(=\frac{1}{f})$ decreases.