We can view a wave as a snapshot of a periodic pattern moving in a certain direction at a constant velocity. We can also view a wave as a pattern in which different locations experience the same temporal oscillation with a certain time lag. Take a wave formed by the spectators in a soccer stadium as an example. If you look at the wave from a helicopter hovering above the stadium, you will see a periodic pattern of people’s hands going around the stadium. This is the first view. If you look at several spectators next to each other, you will see each of them move their hands up and down at the same rate with a certain delay. When the first person in the group raises his or her hands at the highest position, the hands of the person on the left are about to reach the highest position, reaching the highest position a few seconds later. This is the second view of the wave. In this case, the wave travels toward the right of this group of spectators.