more common expression for the group velocity is obtained by introducing the wavevector k:
:We notice that for small changes ??, the magnitude of the corresponding small change in wavevector, say ?k, is:
:so the group velocity can be rewritten as:
:

where ? is the frequency in radians/s: ? = 2f. In all media, frequency and wavevector are related by a dispersion relation, ? = ?(k), and the group velocity can be written:
:In a medium such as classical vacuum