An evanescent wave is an electromagnetic wave that decays exponentially with distance. Evanescent waves are observed in total internal reflection.
The effect has been used to exert radiation pressure on small particles in order to trap them for experimentation, or to cool them to very low temperatures, and to illuminate very small objects such as biological cells for microscopy (as in the total internal reflection fluorescence microscope). The evanescent wave from an optical fiber can be used in a gas sensor.
In optics, evanescent waves are formed when sinusoidal waves are (internally) reflected off an interface at an angle greater than the critical angle so that total internal reflection occurs.
"Evanescent" means "tending to vanish", which is appropriate because the intensity of evanescent waves decays exponentially (rather than sinusoidally) with the distance from the interface at which they are formed.
Mathematically, evanescent waves are characterized by having an imaginary value of the wavenumber, k.
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