Question

If a convex lens is placed in water, its focal length \dots

• A. increases
• B. decreases
• C. does not change
• D. None

Hint:

Greater surrounding refractive index means weaker bending of light by the lens, resulting in larger focal length.

Answer 1

The Correct Option is A

Concept: The focal length of a lens depends upon the refractive index of the lens material relative to the surrounding medium. According to the lens maker's formula: \[ \frac{1}{f} = \left( \frac{\mu_{\text{lens}}}{\mu_{\text{medium}}} -1 \right) \left( \frac{1}{R_1} - \frac{1}{R_2} \right) \] Explanation: Normally, a convex lens is placed in air. Air has a very small refractive index compared to glass. When the lens is placed in water:
• The refractive index of the surrounding medium increases.
• The difference between refractive index of glass and water decreases.
• Hence, the converging power of the lens decreases. Since power decreases, focal length increases because: \[ P = \frac{1}{f} \] Thus, a convex lens becomes less powerful in water and its focal length increases.