The index of refraction for water is about 4 3. What happens as a beam of light travels from air into water?

Its speed decreases.

Explanation:

When light enters into water, its speed decreases (as well as its frequency). This is the reason why refraction (the bending of light when it changes media) occurs.

This can be proven mathematically.

Given that the refractive index of water is 4/3, we know that refractive index is the ratio of the speed of light in a vacuum to the speed of light in another medium.

Therefore,

4/3 = (speed of light in vacuum) / (speed of light in water)

Speed of light in a vacuum is 3 * 10^8. Hence,

Speed of light in water is:

v = (3 * 10^8) / (4/3)

v = 2.25 * 10^8 m/s

As we can see, the speed of light decreases in water.