Total external reflection: Difference between revisions

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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 {{webarchiveCite 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 |date=12url-13-2016|titlestatus=X-raybot: unknown Telescopes}}</ref>

The critical angle of total external reflection can be calculated as:<ref>Y. Chen, "General law of refraction" https://assets-eu.researchsquare.com/files/rs-4783430/v1_covered_eebd8628-fdf9-4366-bfaa-bef42f6128d5.pdf</ref>

:<math display="block">\theta_\mathrm{c} =\arcsin\!\left(\frac{|\underline{n_2}|}{|\underline{n_1}|}\right)\!= \arcsin\!\left(\frac{\sqrt{n_2^2+\kappa_2^2}}{\sqrt{n_1^2+\kappa_1^2}}\right)\!.</math>

where <math>\underline{n}</math> is the complex refractive index, and <math>\kappa</math> is the extinction coefficient.

==References==

{{reflist}}