Phase velocity

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Gravity waves on the ocean surface over deep water. The red dot moves with the phase velocity. The green dots propagate with the group velocity. The phase velocity here is twice the group velocity.

The phase velocity of a wave is the rate at which the phase of any one frequency component of the wave travels.

Any given phase of the wave (for example, the crest) will appear to travel at the phase velocity. The phase velocity is given in terms of the wavelength λ (lambda) and wave period T as