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

The Correct Option is B

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.