Light moves in a vacuum at a speed of . In transparent materials it moves at a speed less than by a factor which is called the refractive index of the material:
Often the refractive index takes the form
where is the wavenumber and and are constants characteristic of the material. The angular frequency of light in a transparent medium is thus
so the frequency increases slightly with increasing k. Typically, when k is near kR, the material becomes opaque.
Ultimately, this is due to resonance between the light and the atoms of the materials.