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

1. remain unchanged
2. become zero
3. become infinite
4. become small, but non-zero

Correct answer: become infinite

Solution

When the refractive index of the liquid matches that of the lens, the lens effectively loses its ability to bend light (no refraction occurs). As a result, the lens behaves as if it is not present, and its focal length becomes infinite.

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