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Group velocity
Physical quantity / From Wikipedia, the free encyclopedia
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The group velocity of a wave is the velocity with which the overall envelope shape of the wave's amplitudes—known as the modulation or envelope of the wave—propagates through space.
![Thumb image](http://upload.wikimedia.org/wikipedia/commons/b/bd/Wave_group.gif)
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![Thumb image](http://upload.wikimedia.org/wikipedia/commons/thumb/c/c7/Wave_opposite-group-phase-velocity.gif/220px-Wave_opposite-group-phase-velocity.gif)
For example, if a stone is thrown into the middle of a very still pond, a circular pattern of waves with a quiescent center appears in the water, also known as a capillary wave. The expanding ring of waves is the wave group or wave packet, within which one can discern individual waves that travel faster than the group as a whole. The amplitudes of the individual waves grow as they emerge from the trailing edge of the group and diminish as they approach the leading edge of the group.