Editing Refractive index (section)

===Optical path length===
[[File:Soap bubble sky.jpg|thumb|alt=Soap bubble|The colors of a [[soap bubble]] are determined by the [[optical path length]] through the thin soap film in a phenomenon called [[thin-film interference]].]]

[[Optical path length]] (OPL) is the product of the geometric length {{mvar|d}} of the path light follows through a system, and the index of refraction of the medium through which it propagates,<ref>R. Paschotta, article on [https://www.rp-photonics.com/optical_thickness.html optical thickness] {{webarchive|url=https://web.archive.org/web/20150322115346/http://www.rp-photonics.com/optical_thickness.html |date=2015-03-22 }} in the [https://www.rp-photonics.com/encyclopedia.html Encyclopedia of Laser Physics and Technology] {{webarchive|url=https://web.archive.org/web/20150813044135/http://www.rp-photonics.com/encyclopedia.html |date=2015-08-13 }}, accessed on 2014-09-08</ref>
<math display="text">\text{OPL} = nd.</math>
This is an important concept in optics because it determines the [[phase (waves)|phase]] of the light and governs [[interference (wave propagation)|interference]] and [[diffraction]] of light as it propagates. According to [[Fermat's principle]], light rays can be characterized as those curves that [[Mathematical optimization|optimize]] the optical path length.<ref name=Hecht/>{{rp|68–69}}