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:

$$c = c_{vac} / n .$$ (2.14)

Often the refractive index takes the form

$$n^2 \approx 1 + \frac{A}{1 - (k/k_R )^2} ,$$ (2.15)

where $k$ is the wavenumber and $k_R$ and $A$ are positive constants characteristic of the material. The angular frequency of light in a transparent medium is thus

$$\omega = kc = k c_{vac} /n .$$ (2.16)