The velocity at which a single frequency of a wave group or a phase—or part such as the crest or trough—of a wave group travels through a medium. The phase velocity (vp) is defined by a wavelength (λ) and frequency (f) and given by vp = λ × f.
vp = vg + λ (∂vp/∂λ) = vg − f (∂vp/∂f)
Relation of phase velocity to group velocity. As a wave travels through a medium, its energy moves at the group velocity (vg) and its individual phases, or components, move at their phase velocity (vp). The wave changes shape with distance as each frequency (f), or wavelength (λ), component moves at its separate phase velocity through the phenomenon of dispersion. Relative to the group velocity, each component moves with faster or slower phase velocity, depending on how phase velocity changes with wavelength or frequency.
Slowness is the reciprocal of phase velocity.