Question

Does a swimmer underwater observe the same color or a different color for this light?

Answer 1

Answer: A swimmer underwater observes the same color because the frequency does not change.

Explanation:

When a swimmer is underwater and a light pasess from air to water, there is a change in the medium and its index of refraction, hence the light is refracted. This means it changes its direction.

Nevertheless, in this process the refracted ray of light does not change its frequency $f$, because frequency is:

$f=\frac{1}{T}$

Where $T$ is the period of the wave and this remains unchanged, hence the frequency, as well.

So, as the frequency does not change and the color of light depends on frequency, the color of the light remains the same.