One wave has and , while the other has and . Thus, and and we have . Notice that the phase speed for the first sine wave is and for the second wave is . Thus, in this case. Figure 1.14 shows a non-dispersive case in which the phase speed equals the group velocity.
Large displacements indicate the location of wave packets.
The positions of waves and wave packets at any given time may therefore be determined by drawing a horizontal line across the graph at the desired time and examining the variations in wave displacement along this line. The crests of the waves are indicated by regions of short vertical lines. Notice that as time increases, the crests move to the right. This corresponds to the motion of the waves within the wave packets. Note also that the wave packets, i. e., the broad regions of large positive and negative amplitudes, move to the right with increasing time as well.