Question

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

• A. become zero
• B. become infinity
• C. reduce
• D. increase

Hint:

When the refractive index of the surrounding medium equals the refractive index of the lens material, the lens loses its power to focus, leading to a focal length of infinity.

Answer 1

The Correct Option is B

Step 1: Refractive index and focal length.
The focal length \( f \) of a lens is related to its refractive index \( n \) and the refractive index of the surrounding medium. When a lens is immersed in a liquid with the same refractive index as the lens, the refraction at the surfaces of the lens becomes nullified. This leads to the lens losing its focusing ability.
Step 2: Conclusion.
When the refractive index of the surrounding medium matches that of the lens material, the focal length of the lens becomes infinity.
\[ \boxed{\text{Correct Answer: Become infinity}} \]