Editing Coherence (physics) (section)

== Coherence and correlation ==
The coherence of two waves expresses how well correlated the waves are as quantified by the [[cross-correlation]] function.<ref name="winter">{{cite journal
|author=Rolf G. Winter
|author2=Aephraim M. Steinberg
|year=2008
|title=Coherence
|url=http://accessscience.com/content/Coherence/146900
|journal=AccessScience
|publisher=[[McGraw-Hill]]
|doi=10.1036/1097-8542.146900
}}</ref><ref name="BornWolf">{{cite book
|author=M.Born
|author2=E. Wolf
|year=1999
|title=[[Principles of Optics]]
|edition=7th
|publisher=[[Cambridge University Press]]
|isbn= 978-0-521-64222-4
}}</ref><ref name="Loudon">{{cite book
|author=Loudon, Rodney
|year=2000
|title=The Quantum Theory of Light
|publisher=[[Oxford University Press]]
|isbn=978-0-19-850177-0
}}</ref><ref name="mandel">{{cite book
|author=Leonard Mandel
|author2=Emil Wolf
|year=1995
|title=Optical Coherence and Quantum Optics
|publisher=[[Cambridge University Press]]
|isbn=978-0-521-41711-2
}}</ref><ref name="mar">{{cite book
|author=Arvind Marathay
|year=1982
|title=Elements of Optical Coherence Theory
|publisher=[[John Wiley & Sons]]
|isbn=978-0-471-56789-9
}}</ref> Cross-correlation quantifies the ability to predict the phase of the second wave by knowing the phase of the first. As an example, consider two waves perfectly correlated for all times (by using a monochromatic light source). At any time, the phase difference between the two waves will be constant. If, when they are combined, they exhibit perfect constructive interference, perfect destructive interference, or something in-between but with constant phase difference, then it follows that they are perfectly coherent. As will be discussed below, the second wave need not be a separate entity. It could be the first wave at a different time or position. In this case, the measure of correlation is the [[autocorrelation]] function (sometimes called ''self-coherence''). Degree of correlation involves correlation functions.{{R|Hecht2002|page=545-550}}