© Adam Kleppner.
There is no better way to study waves than by tossing pebbles into a pond on a calm sunny day. A single pebble causes a spreading circular wave, as shown here. This wave only continues for a few wavelengths, but if the disturbance were periodic, a steady stream of waves would be generated. Clearly, the energy of the splash is being carried outward in all directions. Tossing a couple of pebbles nearby creates a set of circular waves that magically pass through each other, demonstrating the principle of superposition that plays an almost magical role in quantum mechanics. If the wave amplitude becomes very high, the waves interact with each other and there is a breakdown of superposition. (Unit: 5)