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.
The refractive index of a material is a measure of the change in the speed of light as it passes from a vacuum (or air as an approximation) into the material.
In the equation above:
The bigger the refractive index, , the slower the light travels in that material - ie the smaller is.
Light of frequency travels at a speed of in diamond.
Calculate the refractive index of diamond for this colour of light.
Refractive index of diamond for this colour of light
Now state the frequency of this light in air.
The frequency of this light in air . (Frequency does not change when light passes from one material to another)