This document discusses coherent and incoherent waves, and the addition of waves through interference. It provides the following key points: - Coherent waves have the same frequency and constant phase difference, while incoherent waves have random frequencies and phase differences. - When two coherent waves travel through the same medium, their displacements add through superposition to form a resultant wave with displacement equal to the sum of the individual waves. - Constructive interference occurs when two waves are coherent and in phase, doubling the amplitude and quadrupling the intensity compared to the individual waves. Destructive interference can also occur when waves are out of phase.