Superposition and Standing Waves in Physics

Waves do not collide, they pass through one another and add up while they overlap. That simple rule produces two of the most useful effects in physics. Two waves arriving in step can double their height, and two arriving out of step can cancel completely, which is the whole basis of interference. Trap a wave between two fixed ends and the reflections keep meeting it, so only a few wavelengths survive the round trip and the rest die away. Those survivors are what we hear as the notes of a guitar string or an organ pipe. The blocks give the addition rule, the interference conditions, the standing wave patterns and their allowed frequencies, and the slow throb of two notes played slightly apart.