Verified answer

The speed of light is determined by the medium (material) through which the light is travelling. Light travels faster in a vacuum than it does in any other medium.

Light changes speed as it passes from one medium to another. This is called refraction.

The frequency of light does not change as it refracts.

Refractive index of a material is a measure of the change in speed of light as it passes from a vacuum (or air as an approximation) into the material.

n = {{\nu _1 } \over {\nu _2 }}

In the equation above, v1 is the speed of light in a vacuum. The bigger the refractive index the slower the light travels in that material - i.e. the smaller v2 is.