Editing Coherence (physics) (section)

== Examples of wave-like states ==
These states are unified by the fact that their behavior is described by a [[wave equation]] or some generalization thereof.
*Waves in a rope (up and down) or [[slinky]] (compression and expansion)
*[[Surface waves]] in a liquid
*[[Electromagnetic]] signals (fields) in [[transmission line]]s
*[[Sound]]
*[[Radio wave]]s and [[microwaves]]
*[[Light wave]]s ([[optics]])
*[[Matter wave]]s associated with, for examples, [[electron]]s and [[atom]]s

In system with macroscopic waves, one can measure the wave directly. Consequently, its correlation with another wave can simply be calculated. However, in optics one cannot measure the [[electric field]] directly as it oscillates much faster than any detector's time resolution.<ref>{{cite journal
 |last1=Peng |first1=J.-L.
 |last2=Liu |first2=T.-A.
 |last3=Shu |first3=R.-H.
 |year=2008
 |title=Optical frequency counter based on two mode-locked fiber laser combs
 |journal=[[Applied Physics B]]
 |volume=92 |issue=4 |pages=513
 |bibcode=2008ApPhB..92..513P
 |doi=10.1007/s00340-008-3111-6
|s2cid=121675431
 }}</ref> Instead, one measures the [[intensity (physics)|intensity]] of the light. Most of the concepts involving coherence which will be introduced below were developed in the field of optics and then used in other fields. Therefore, many of the standard measurements of coherence are indirect measurements, even in fields where the wave can be measured directly.