Question

A convex lens is dipped in a liquid whose refractive index is equal to the refractive index of the lens. Then what is its focal length?

• A. Focal length will become zero
• B. Focal length will become infinite
• C. Focal length will reduce, but not become zero
• D. Remains unchanged

Hint:

When the refractive index of the lens material and the surrounding medium are equal, the lens loses its focusing ability, and its focal length becomes infinite.

Answer 1

The Correct Option is B

Step 1: Understand the concept of focal length.
The focal length \( f \) of a lens depends on the refractive index of the lens material relative to the surrounding medium.
The formula for focal length in terms of refractive index is: \[ \frac{1}{f} = \left( \frac{n_2 - n_1}{R} \right) \] where \( n_2 \) is the refractive index of the lens and \( n_1 \) is the refractive index of the surrounding medium.
Step 2: Focal length in this case.
If the refractive index of the liquid is equal to the refractive index of the lens, the difference between \( n_2 \) and \( n_1 \) becomes zero.
As a result, the focal length becomes infinite, and the lens will no longer converge or diverge light.