Editing Photon (section)

== In matter ==
{{See also|Refractive index|Group velocity|Photochemistry}}

Light that travels through transparent matter does so at a lower speed than ''c'', the speed of light in vacuum. The factor by which the speed is decreased is called the [[refractive index]] of the material. In a classical wave picture, the slowing can be explained by the light inducing [[electric polarization]] in the matter, the polarized matter radiating new light, and that new light interfering with the original light wave to form a delayed wave. In a particle picture, the slowing can instead be described as a blending of the photon with quantum excitations of the matter to produce [[quasi-particle]]s known as [[polariton|polaritons]]. Polaritons have a nonzero [[effective mass (solid-state physics)|effective mass]], which means that they cannot travel at ''c''. Light of different frequencies may travel through matter at [[variable speed of light|different speeds]]; this is called [[dispersion (optics)|dispersion]] (not to be confused with scattering). In some cases, it can result in [[slow light|extremely slow speeds of light]] in matter. The effects of photon interactions with other quasi-particles may be observed directly in [[Raman scattering]] and [[Brillouin scattering]].<ref>Polaritons section 10.10.1, Raman and Brillouin scattering section 10.11.3 in {{Cite book |last1=Patterson |first1=J. D. |title=Solid-State Physics: Introduction to the Theory |last2=Bailey |first2=B. C. |publisher=[[Springer Science+Business Media|Springer]] |year=2007 |isbn=978-3-540-24115-7 |language=en}}</ref>

Photons can be scattered by matter. For example, photons scatter so many times in the solar [[radiative zone]] after leaving the [[Solar core|core of the Sun]] that [[radiant energy]] takes about a million years to reach the [[convection zone]].<ref>{{Cite web |title=The Solar Interior |url=https://solarscience.msfc.nasa.gov/interior.shtml |work=Marshall Space Flight Center: Solar Physics |publisher=National Aeronautics and Space Commission |access-date=4 December 2024}}</ref> However, photons emitted from the sun's [[photosphere]] take only 8.3 minutes to reach Earth.<ref>{{Cite book |last1 = Koupelis |first1 = Theo |last2 = Kuhn |first2 = Karl F. |year = 2007 |url = https://books.google.com/books?id=6rTttN4ZdyoC&pg=PA102 |title = In Quest of the Universe |page = 102 |publisher = Jones and Bartlett Canada |isbn = 9780763743871 |access-date = 2020-11-29 |archive-date = 2024-05-12 |archive-url = https://web.archive.org/web/20240512231402/https://books.google.com/books?id=6rTttN4ZdyoC&pg=PA102#v=onepage&q&f=false |url-status = live }}</ref>

Photons can also be [[absorption (electromagnetic radiation)|absorbed]] by nuclei, atoms or molecules, provoking transitions between their [[energy level]]s. A classic example is the molecular transition of [[retinal]] (C<sub>20</sub>H<sub>28</sub>O), which is responsible for [[Visual perception|vision]], as discovered in 1958 by Nobel laureate [[biochemist]] [[George Wald]] and co-workers. The absorption provokes a [[cis–trans]] [[isomerization]] that, in combination with other such transitions, is transduced into nerve impulses. The absorption of photons can even break chemical bonds, as in the [[photodissociation]] of [[chlorine]]; this is the subject of [[photochemistry]].<ref>E.g. section 11-5 C in {{Cite book |last1=Pine |first1=S. H. |title=Organic Chemistry |last2=Hendrickson |first2=J. B. |last3=Cram |first3=D. J. |last4=Hammond |first4=G. S. |publisher=McGraw-Hill |year=1980 |isbn=978-0-07-050115-7 |edition=4th |language=en}}</ref><ref>Nobel lecture given by G. Wald on December 12, 1967, online at nobelprize.org: [http://nobelprize.org/nobel_prizes/medicine/laureates/1967/wald-lecture.html The Molecular Basis of Visual Excitation] {{Webarchive|url=https://web.archive.org/web/20160423182216/http://www.nobelprize.org/nobel_prizes/medicine/laureates/1967/wald-lecture.html |date=2016-04-23 }}.</ref>