Wave phenomena occur almost anywhere there is periodic motion. We have already encountered such periodic motion in the back-and-forth movement of pendulums and masses on a spring and with the cyclic orbits of objects in a gravitational field. The physics of waves is also central in explaining how light and sound work. Anything from a violin string to a drum skin to a wine glass can make a sound, suggesting that there are few things in the world that cannot produce wave phenomena. We find waves in the air, in our bodies, in earthquakes, in computers—and, if we’re surfers, at the beach.