Wave interference between two point sources of light.

Sinusoidal light waves add or subtract to produce the pretty patterns seen below. Thomas Young’s observation of this behavior in the early 1800s eventually came to be seen as slam-dunk evidence for the wave-nature of light, forcing a particle-theory of light (Newton, for example, thought that light was made up of tiny corpuscles) into the background for the rest of the century. Enjoy.