so in general, the phase velocity and the group velocity depend on specific medium and frequency. The ratio between the speed of light c and the phase velocity vp is known as the refractive index, . In this way, we can obtain another form for group velocity for electromagnetics. Writing , a quick way to derive this form is to observe We can then rearrange the above to obtain From this formula, we see that the group velocity is equal to the phase velocity only when the refractive index is a constant . When this occurs, the medium is called non-dispersive, as opposed to dispersive, where various properties of the medium depend on the frequency . The relation is known as the dispersion relation of the medium. See also Cherenkov radiation Dispersion (optics) Group velocity Propagation delay Shear wave splitting Wave propagation Wave propagation speed Planck constant Speed of light Matter wave#Phase velocity References Footnotes Bibliography Crawford jr., Frank S. (1968). Waves (Berkeley Physics Course, Vol. 3), McGraw-Hill, Free online version Wave mechanics