Light (or, more generally, electromagnetic radiation) is a wave phenomenon, with wave crests and troughs, maxima and minima of the electric and magnetic fields.

This has characteristic consequences: For such waves, there is a phenomenon called interference. One case is shown in the drawing at the top: There, two waves, one drawn blue, the other red, are superposed in a way that the crests of the one are at the same location as the crests of the other, and similarly for the wave troughs. The result is the wave drawn in violet: a wave for which the crests are substantially higher, the troughs deeper than for the partial waves - an example for constructive interference.