A standing wave is the wave formed by two waves of equal amplitude traveling in opposite directions. According to the principle of superposition discussed earlier, the displacement of the standing wave at a particular point on the string at some particular instant in time is just the sum of the two individual displacements of the two traveling waves. The formation of a standing wave is represented graphically in Figure 11 below. The first graph shows where the wave is at a time equal to one period after the wave machine was turned on. Notice that after one period, the machine has generated one complete wavelength.
Subsequent graphs show the wave each quarter-period later than the previous graph.
Now it starts to get reflected. In graphs four through eight, the reflected wave is drawn as a light grey line , and the original wave that would have continued to travel to the right is drawn as a dark grey line . The principle of superposition tells us that the resultant wave is obtained by adding the two traveling waves at each point along the string. This resultant wave is shown as the solid black line in Figure 11. The blue wave at the top of Figure 11B is the standing wave .