Editing Wave (section)

=== Phase velocity and group velocity ===
{{Main|Phase velocity|Group velocity}}
{{See also|Envelope (waves)#Phase and group velocity}}
[[Image:Wave group.gif|class=skin-invert-image|thumb|The red square moves with the [[phase velocity]], while the green circles propagate with the [[group velocity]].|left]]

There are two velocities that are associated with waves, the [[phase velocity]] and the [[group velocity]].

Phase velocity is the rate at which the [[phase (waves)|phase]] of the wave [[Wave propagation|propagates in space]]: any given phase of the wave (for example, the [[crest (physics)|crest]]) will appear to travel at the phase velocity. The phase velocity is given in terms of the [[wavelength]] {{mvar|λ}} (lambda) and [[Wave period|period]] {{mvar|T}} as
<math display="block">v_\mathrm{p} = \frac{\lambda}{T}.</math>

[[Image:Wave opposite-group-phase-velocity.gif|class=skin-invert-image|thumb|right|A wave with the group and phase velocities going in different directions]]

Group velocity is a property of waves that have a defined envelope, measuring propagation through space (that is, phase velocity) of the overall shape of the waves' amplitudes—modulation or envelope of the wave.