Editing Standing wave (section)

=== Visible light ===
Standing waves are also observed in optical media such as [[Waveguide (optics)|optical waveguides]] and [[optical cavity|optical cavities]]. [[Laser]]s use optical cavities in the form of a pair of facing mirrors, which constitute a [[Fabry–Pérot interferometer]]. The [[Active laser medium|gain medium]] in the cavity (such as a [[crystal]]) emits light [[Coherence (physics)|coherently]], exciting standing waves of light in the cavity.<ref>{{cite book |first1=Rayf |last1=Shiell |first2=Iain |last2=McNab |date=2024 |title= Pedrottis' Introduction to Optics |publisher=[[Cambridge University Press]] |edition=4 |isbn= 9781316518625}}</ref> The wavelength of light is very short (in the range of [[nanometer]]s, 10<sup>−9</sup> m) so the standing waves are microscopic in size. One use for standing light waves is to measure small distances, using [[optical flat]]s.