Editing Coherence (physics) (section)

{{Short description|Potential for two waves to interfere}}
{{Other uses|Coherence (disambiguation){{!}}Coherence}}
{{Use American English|date = March 2019}}
'''Coherence''' expresses the potential for two waves to [[Wave interference|interfere]]. Two [[Monochromatic radiation|monochromatic beams]] from a single source always interfere.<ref name="BornWolf" />{{rp|p=286}} Wave sources are not strictly monochromatic: they may be ''partly coherent''. 

When interfering, two waves add together to create a wave of greater amplitude than either one (constructive [[Wave interference|interference]]) or subtract from each other to create a wave of minima which may be zero<ref name="BornWolf" />{{rp|p=286}} (destructive interference), depending on their relative [[phase (waves)|phase]].  Constructive or destructive interference are limit cases, and two waves always interfere, even if the result of the addition is complicated or not remarkable.

Two waves with constant relative phase will be coherent.<ref name="pashcotta">{{Cite web |last=Rudiger |first=Paschotta |title=Article on Coherence in the RP Photonics Encyclopedia |url=https://www.rp-photonics.com/encyclopedia.html |access-date=2023-06-07 |website=RP Photonics Encyclopedia}}</ref> The amount of coherence can readily be measured by the [[interference visibility]], which looks at the size of the interference fringes relative to the input waves (as the phase offset is varied); a precise mathematical definition of the [[degree of coherence]] is given by means of correlation functions. More broadly, coherence describes the statistical similarity of a field, such as an electromagnetic field or quantum wave packet, at different points in space or time.<ref>{{Cite book | title=Introduction to the theory of coherence and polarization of light | first = Emil | last = Wolf | date = 2007 | publisher=Cambridge University Press | isbn=978-0-521-82211-4 | location=Cambridge | oclc=149011826}}</ref>