Dielectric mirrors are glass or other substrates on which one or more layers of dielectric material are deposited, to form an optical coating. A very complex dielectric mirror can reflect up to 99.999% of the light incident upon it, for a narrow range of wavelengths and angles. A simpler mirror may reflect 99.9% of the light, but may cover a broader range of wavelengths.
Almost any dielectric material can act as a perfect mirror through total internal reflection. This effect only occurs at shallow angles, however, and only for light inside the material. The effect happens when light goes from a medium with a higher index of refraction to one with a lower value (like air).
A new type of dielectric "perfect mirror" was developed in 1998 by researchers at MIT. These unusual mirrors are very efficient reflectors over a broad range of angles and wavelengths, and are insensitive to polarization. A version of the perfect mirror that was developed at MIT for military use is used by OmniGuide in laser surgery. 
- "MIT researchers create a 'perfect mirror'". MIT. 27 November 1998. Retrieved 2012-11-18.
- "A Dielectric Omnidirectional Reflector". Science. November 26, 1998. Retrieved 2008-09-22.
- "From military device to life-saving surgery tool". CNN. December 19, 2008. Retrieved 2008-12-19.
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