Image illustrating the principle of Thomas Young's classic double slit experiment. A flat wave is directed at a screen with two gaps. The gaps act as new wave sources as the wavefronts bend to create two sets of circular waves radiating outwards. These tow new waves create an interference pattern as they superimpose on each other. The waves sum together producing higher peaks and deeper toughs at some points, whilst cancelling each other out at other points. This produces the classic interference pattern.