Total external reflection: Difference between revisions

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{{Short description|Situation or phenomena, When light bounces off a material with a low index of refraction}}

'''Total external reflection''' is an optical phenomenon where electromagnetic 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 emerges from under water. The optically more dense material (water in this case) is the "internal" medium. For visible light, water has an index of refraction of 1.33 and for air it is very close to 1. For vacuum the index of refraction is exactly 1 for all wavelengths.

{{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 externalinternal reflection''' isdescribes anthe opticalfact phenomenon where electromagneticthat 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 emergesstarts from underin water. Theand opticallybounces more dense material (water in this case) isoff the "internal" medium. For visible light, water has an index of refraction of 1.33 and for -to-air it is very close to 1. For vacuum the index of refraction is exactly 1 for all wavelengthsinterface.

For [[X-rays]], however, all materials have indices of refraction slightly below 1. This entails that total reflection of X-rays only can occur when they travel through vacuum and impinge on a surface (at a small glancing angle). Since this kind of total reflection takes place outside of the material it is termed ''total external reflection''.

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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[[Category:Geometrical optics]]

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