Wave groups produced by two harmonic waves of nearly identical frequencies. The upper diagram shows the waves as they are seen by an observer. The lower diagram shows the same waves but includes the envelope of the wave groups or packets as a red line.
The waves (yellow lines) travel at wave speed c; the wave groups or packets (red lines) travel at group velocity cg. The packets transport the energy contained in the wave field, which means that the energy also travels at group velocity cg.
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