This document discusses interference, which occurs when two or more waves overlap. There are two types of interference: constructive and destructive. Constructive interference occurs when waves are displaced in the same direction and amplitudes add, while destructive interference occurs when they are displaced in opposite directions and amplitudes subtract. The document provides examples of interference in light, radio, acoustic, and water waves. It describes Young's double-slit experiment, which demonstrated that light behaves as waves that can interfere and was evidence against the particle theory of light.