The "velocity" of a wave is a more subtle concept. In fact, there are several kinds of wave velocities, the most common of which are group velocity and phase velocity. Group velocity is the speed of energy and information propagation and, for an electromagnetic wave, must be less than the speed of lightc. However, the velocity at which the phase of the wave travels can be greater than the group velocity. The relationship between the two is called the dispersion relationship.

For a general wave of wavelength and frequency, the group velocity is given by

(1)

For a sinusoidal wave,

			(2)
			(3)

(4)

and

(5)

P-Wave, S-Wave, Speed of Sound, String, Velocity, Wave