FIGURE 17.3 is a time-lapse photograph of a standing wave on a vibrating string. It’s not obvious from the photograph, but this is actually a superposition of two waves. To understand this, consider two sinusoidal waves with the same frequency, wavelength, and amplitude traveling in opposite directions. For example, FIGURE 17.4a shows two waves on a string, and FIGURE 17.4b shows nine snapshot graphs, at intervals of $\frac{1}{8}T$. The dots identify two of the crests to help you visualize the wave movement.

FIGURE 17.3 A vibrating string is an example of a standing wave.

FIGURE 17.4 The superposition of two sinusoidal waves traveling in opposite directions.

At each point, the net displacement—the superposition—is found by adding the red displacement and the green displacement. FIGURE 17.4c shows the result. It is the wave you would actually observe. The blue dot shows that the blue wave is moving neither right nor left. The wave of Figure 17.4c is called a standing wave because the crests and troughs “stand in place” as the wave oscillates.