Total external reflection is a phenomenon traditionally involving
X-rays
An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 Picometre, picometers to 10 Nanometre, nanometers, corresponding to frequency, ...
, but in principle any type of electromagnetic or other wave, closely related to
total internal reflection
Total internal reflection (TIR) is the optical phenomenon in which waves arriving at the interface (boundary) from one medium to another (e.g., from water to air) are not refracted into the second ("external") medium, but completely reflected b ...
.
Total internal reflection
Total internal reflection (TIR) is the optical phenomenon in which waves arriving at the interface (boundary) from one medium to another (e.g., from water to air) are not refracted into the second ("external") medium, but completely reflected b ...
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
Snell's law (also known as Snell–Descartes law and ibn-Sahl law and the law of refraction) is a formula used to describe the relationship between the angles of incidence and refraction, when referring to light or other waves passing through ...
).
Total internal reflection
Total internal reflection (TIR) is the optical phenomenon in which waves arriving at the interface (boundary) from one medium to another (e.g., from water to air) are not refracted into the second ("external") medium, but completely reflected b ...
occurs when the first medium has a larger
refractive index
In optics, the refractive index (or refraction index) of an optical medium is a dimensionless number that gives the indication of the light bending ability of that medium.
The refractive index determines how much the path of light is bent, or ...
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. This makes it possible to focus X-rays.
[See, for example, NASA ]
References
Geometrical optics
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