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To understand dispersion, we need to explain what is the index of refraction of a material, a concept which was introduced earlier.
The index of refraction (n) of a material is defined as the speed of light in the vacuum (c) divided by the speed of light in that material (v).
We know that the speed of a wave
(v) such as light is
equal to the frequency
(f) multiplied by its
wavelength
.
The frequency of light remains
unchanged when passing from one
medium to another.
The index of refraction can be written as:
where 
is the wavelength
of the light in the vacuum and
the wavelength of the light in the material.
What does this all mean?
The wavelength of light in a medium depends on the refractive index of the medium.
Let's go back to the fizzicks of rainbows...