Editing Refractive index (section)

===Total internal reflection===
{{Main|Total internal reflection}}

[[File:Total internal reflection of Chelonia mydas.jpg|thumb|alt=A sea turtle being reflected in the water surface above|[[Total internal reflection]] can be seen at the air-water boundary.]]
If there is no angle {{math|''θ''{{sub|2}}}} fulfilling Snell's law, i.e.,
<math display="block">\frac{n_1}{n_2} \sin \theta_1 > 1,</math>
the light cannot be transmitted and will instead undergo [[total internal reflection]].<ref name = bornwolf />{{rp|49–50}} This occurs only when going to a less optically dense material, i.e., one with lower refractive index. To get total internal reflection the angles of incidence {{math|''θ''{{sub|1}}}} must be larger than the critical angle<ref>{{cite encyclopedia |first=R. |last=Paschotta |url=https://www.rp-photonics.com/total_internal_reflection.html|title=Total Internal Reflection|encyclopedia=RP Photonics Encyclopedia |access-date=2015-08-16 |url-status=live |archive-url=https://web.archive.org/web/20150628175307/https://www.rp-photonics.com/total_internal_reflection.html |archive-date=2015-06-28 }}</ref>
<math display="block">\theta_\mathrm{c} = \arcsin\!\left(\frac{n_2}{n_1}\right)\!.</math>