group velocity

group velocity [¦grüp və′läs·əd·ē]

(physics)

The velocity of the envelope of a group of interfering waves having slightly different frequencies and phase velocities.

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Group velocity

The velocity of propagation of a group of waves forming a wave packet; also, the velocity of energy flow in a traveling wave or wave packet. The pure sine waves used to define phase velocity v_p do not ever really exist, for they would require infinite extent. What do exist are groups of waves, wave packets, which are combined disturbances of a group of sine waves having a range of frequencies and wavelengths. Good approximations to pure sine waves exist, provided the extent of the media is very large in comparison with the wavelength of the sine wave. In nondispersive media, pure sine waves of different frequencies all travel at the same speed v_p, and any wave packet retains its shape as it propagates. In this case, the group velocity v_g is the same as v_p. But if there is dispersion, the wave packet changes shape as it moves, because each different frequency which makes up the packet moves with a different phase velocity. If v_p is frequency-dependent, then v_g is not equal to v_p. See Phase velocity, Sine wave, Wave motion