Question

A convex lens of glass is immersed in water compared to its power in air, its power in water will

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

Answer 1

Answer: (B)

decrease

Answer 2

The correct option is: (B): decrease.
When a convex lens is immersed in water, its focal length increases, resulting in a decrease in its power.
According to the lens maker's formula:
$\frac{1}{f}=(\frac{μ_2}{​μ_1}−1)(\frac{1}{R_1}-\frac{1}{R_2})$
When the lens is immersed in water, the refractive index of the medium μ1 increases from 1 to $\frac{4}{3}$​ (the refractive index of water). As a result, the $\frac{μ_2}{μ_1}$​​ term decreases, causing the $(\frac{μ_2}{μ_1}-1)$ term to decrease, which leads to a decrease in $\frac{1}{f}$ Therefore, the power $(=\frac{1}{f})$ decreases.