Definition: | the velocity vector at a given point and in a given direction of the movement of a wavefront of a sinusoidal travelling wave, defined by a specified phase
NOTE 1 – If no direction is specified, the phase velocity to be considered is that in the direction of propagation.
NOTE 2 – In the direction of propagation, the magnitude of the phase velocity has a minimum value equal to the angular frequency divided by the absolute value of the real component of the wave vector.
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