Editing Refractive index (section)

==Definition==
The '''relative refractive index''' of an optical medium 2 with respect to another reference medium 1 ({{math|''n''<sub>21</sub>}}) is given by the ratio of speed of light in medium 1 to that in medium 2. This can be expressed as follows:
<math display="block">n_{21}=\frac{v_1}{v_2}.</math>
If the reference medium 1 is [[vacuum]], then the refractive index of medium 2 is considered with respect to vacuum. It is simply represented as {{math|''n''<sub>2</sub>}} and is called the '''absolute refractive index''' of medium 2.

The '''absolute refractive index''' ''n'' of an optical medium is defined as the ratio of the [[speed of light]] in vacuum, {{math|1=c = {{val|299792458|u=m/s}}}}, and the [[phase velocity]] {{mvar|v}} of light in the medium,
<math display="block">n=\frac{\mathrm{c}}{v}.</math>
Since {{math|c}} is constant, {{mvar|n}} is inversely proportional to {{mvar|v}}:
<math display="block">n\propto\frac{1}{v}.</math>
The phase velocity is the speed at which the crests or the [[phase (waves)|phase]] of the [[wave]] moves, which may be different from the [[group velocity]], the speed at which the pulse of light or the [[Envelope (waves)|envelope]] of the wave moves.<ref name="Hecht" /> Historically [[air]] at a standardized [[pressure]] and [[temperature]] has been common as a reference medium.