When a plano-concave lens made of glass (index of refraction 1.65) is placed in water (index of refraction 1.33), its focal length
A) increases.
B) decreases.
C) stays the same.
D) becomes zero.

Question

Quizplus · Accepted Answer

The focal length of a lens depends on the ratio of the indices of refraction between the lens material and the surrounding medium. When a plano-concave lens is placed in water, the effective difference in the indices of refraction decreases (from air to water), making the lens less powerful and thus increasing its focal length.

When a Plano-Concave Lens Made of Glass (Index of Refraction