# Why is refractive index equals to speed of light upon phase velocity and not group velocity?

We have been using group velocity for most purposes as its the velocity with which the wave packet travels, they why do we use phase velocity for calculating refractive index.

• It falls out that way from maxwells equations, if you have a non dispersive medium. So it is convenient. If you have a medium then it is not precisely true but often close enough for a lot of calculations. If you have some spectral bandwidth or you are more concerned about the pulse shape then it becomes more important to include the dispersive effects. May 5 at 21:16

Group velocity is $\partial \omega /\partial k$ so it involves the derivative of the refractive index to wavenumber/frequency.