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{{Short description|Situation or phenomena, When light bounces off a material with a low index of refraction}}
{{Refimprove|date=September 2014}}
'''Total external reflection''' is a phenomenon traditionally involving [[X-rays]], but in principle any type of electromagnetic or other wave, closely related to [[total internal reflection]].
Total internal reflection describes the fact that radiation (e.g. visible light) can, at certain angles, be totally reflected from an interface between two media of different [[indices of refraction]] (see [[Snell's law]]). Total internal reflection occurs when the first medium has a larger refractive index than the second medium, for example, light that starts in water and bounces off the water-to-air interface.
Total external reflection is the situation where the light starts in air and vacuum (refractive index 1), and bounces off a material with index of refraction less than 1. For example, in X-rays, the refractive index is frequently slightly less than 1, and therefore total external reflection can happen at a glancing angle. It is called ''external'' because the light bounces off the exterior of the material.<ref>{{cite book|last=Attwood|first=D|title=Soft X-Rays and Extreme Ultraviolet Radiation|year=1999|publisher=Cambridge University Press}}</ref> This makes it possible to focus X-rays.<ref>See, for example, NASA {{Cite web |url=http://imagine.gsfc.nasa.gov/science/toolbox/xray_telescopes1.html |title=X-ray Telescopes |access-date=2020-05-11 |archive-date=2016-12-13 |archive-url=https://web.archive.org/web/20161213145838/http://imagine.gsfc.nasa.gov/science/toolbox/xray_telescopes1.html |url-status=bot: unknown }}</ref>
==References==
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{{DEFAULTSORT:Total External Reflection}}
[[Category:Geometrical optics]]
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