group velocity

(redirected from group velocities)

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 vp 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 vp, and any wave packet retains its shape as it propagates. In this case, the group velocity vg is the same as vp. 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 vp is frequency-dependent, then vg is not equal to vp. See Phase velocity, Sine wave, Wave motion

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.
References in periodicals archive ?
The phase and group velocities of (22) are smaller than the velocity constant c.
The spin occurs at the expense of a small reduction of the phase and group velocities in the direction of propagation.
Therefore given the elastic properties, group velocities of all three modes are determined for a given direction [[Theta].
For the purpose of illustration, phase or group velocities will be shown assuming typical properties for oriented polymer materials, as given in Table 1.
A least-square fit of the N data points to the model consists in minimizing the sum of squares of the deviations between the slownesses of experimental and calculated group velocities, respectively, [Mathematical Expression Omitted] and [Mathematical Expression Omitted], and making adjustable the unknowns [C.
0] = 0 and the phase and group velocities both are equal to c;
thus resulting in axial phase and group velocities, both being equal to v < c .