where m 0 {\displaystyle m_{0}} denotes the particle's rest mass, v {\displaystyle v} its velocity, γ {\displaystyle \gamma } the Lorentz factor, and c {\displaystyle c} the speed of light in a vacuum.[18][19][20] See below for details of the derivation of the de Broglie relations. Group velocity (equal to the particle's speed) should not be confused with phase velocity (equal to the product of the particle's frequency and its wavelength). In the case of a non-dispersive medium, they happen to be equal, but otherwise they are not.