6.3 The law of refraction  (Page 2/7)

The speed of light is now known to great precision. In fact, the speed of light in a vacuum $c$ is so important that it is accepted as one of the basic physical quantities and has the fixed value

where the approximate value of $3\text{.}\text{00}\times {\text{10}}^8\text{m/s}$ is used whenever three-digit accuracy is sufficient. The speed of light through matter is less than it is in a vacuum, because light interacts with atoms in a material. The speed of light depends strongly on the type of material, since its interaction with different atoms, crystal lattices, and other substructures varies. We define the index of refraction     $n$ of a material to be

where $v$ is the observed speed of light in the material. Since the speed of light is always less than $c$ in matter and equals $c$ only in a vacuum, the index of refraction is always greater than or equal to one.

That is, $n\ge 1$ . [link] gives the indices of refraction for some representative substances. The values are listed for a particular wavelength of light, because they vary slightly with wavelength. (This can have important effects, such as colors produced by a prism.) Note that for gases, $n$ is close to 1.0. This seems reasonable, since atoms in gases are widely separated and light travels at $c$ in the vacuum between atoms. It is common to take $n=1$ for gases unless great precision is needed. Although the speed of light $v$ in a medium varies considerably from its value $c$ in a vacuum, it is still a large speed.