Wave Interference
When two waves meet, constructive and destructive
Summary
When two waves overlap at the same point, sometimes they reinforce each other (constructive interference) and sometimes they cancel out (destructive interference). Which one happens is decided by the difference between the paths from the two sources to that point. This is a shared behaviour of all waves, including water, sound and light.
In depth
According to the superposition principle, when two waves overlap in the same medium the result is the sum of the two. If two crests coincide, a bigger crest forms (constructive); if a crest meets a trough, they cancel each other (destructive). Which of these happens at a point depends on the difference in the point's distances from the two sources, the path difference: if the path difference is a whole multiple of the wavelength (nλ) the interference is constructive, and if it is a half-integer multiple ((n+½)λ) it is destructive. In his experiments of 1801–1803, Thomas Young showed that light also produces such a striped pattern; this was one of the strongest pieces of evidence that light is a wave. Today interference underlies many technologies, from anti-reflection coatings to noise-cancelling headphones, and from holograms to the gravitational wave detector LIGO.
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