Waves/Light

Light
Light moves in a vacuum at a speed of $$c_{vac} = 3 \times 10^8 \mbox{ m} \mbox{ s}^{-1}$$. In transparent materials it moves at a speed less than $$c_{vac}$$ by a factor $$n$$ which is called the refractive index of the material:
 * $$c = c_{vac} / n . $$

Often the refractive index takes the form
 * $$n^2 \approx 1 + \frac{A}{1 - (k/k_R )^2} ,$$

where $$k$$ is the wavenumber and $$k_R$$ and $$A$$ are constants characteristic of the material. The angular frequency of light in a transparent medium is thus
 * $$\omega = kc = \frac{k c_{vac}}{n} \approx

\frac{k c_{vac}}{\sqrt{1+A}}(1+ \frac{1}{2}\frac{A}{1+A}\frac{k^2}{k^2_R})$$

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.