Illustration of the double-slit experiment, showing a plane wave (top right) passing through a screen with two gaps. The gaps act as new wave sources and the wavefronts bend to create two sets of circular waves radiating outwards. These two new waves create an interference pattern as they interact. At the point where a wave peak from one hits a wave peak from another, or a trough hits a trough, they reinforce each other; their magnitudes are summed. This is known as constructive interference. Where a trough hits a peak, they cancel each other out. This is known as destructive interference.