A concave, spherical mirror and a thin, converging lens are both measured in air (n=1.0) to have the same focal length, ff. Both the mirror and the lens are then placed under water (n=1.33) and their focal lengths are measured again.

Question

For both the mirror and the lens state whether its focal length under water will be the same, longer or shorter than ff and in both cases give a reason for your answer.

Answer 1

the focal length of the concave mirror stays the same.

The focal length of the converging lens gets shorter.

the index of refraction is not a factor in the concave mirror's focal point. However, it is a factor in the converging lens' focal point, as seen in the Lens Maker's Equation, or Snell's law.