Total internal reflection is the phenomenon that occurs when a propagated wave strikes an average limit at an angle greater than a particular critical angle with respect to the normal to the surface. If the refractive index is lower on the other side of the boundary and the angle of incidence is greater than the critical angle, the wave can not traverse and reflect completely. The critical angle is the angle of incidence above which total internal reflection occurs. This is particularly common as an optical phenomenon, where light waves are involved, but occurs with many types of waves, such as electromagnetic waves in general or sound waves. When a wave reaches a boundary between different materials with different refractive indexes, the wave will generally be partially refracted at the boundary surface and partially reflected. However, if the angle of incidence is higher (ie, the direction of propagation is closer to being parallel to the limit) than the critical angle - the angle of incidence at which the light is refracted in such a way that it travels to Along the boundary - then the wave does not cross the boundary, but instead is reflected fully internally. This can only occur when the wave in a medium with a higher refractive index (n1) reaches a limit with a lower refractive index (n2) medium. For example, it will happen with the light that reaches the air of the glass, but not when reaching the glass of the air.
Under ordinary conditions, the evanescent wave transmits zero net energy through the interface. However, if a third medium with a refractive index greater than the second low index medium is situated at less than several wavelength distances from the interface between the first medium and the second medium, the evanescent wave will be different from the And will pass energy through the second in the third medium. (See evanescent wave coupling.) This process is called "frustrated" total internal reflection (FTIR) and is very similar to the quantum tunnel. The quantum tunnel model is mathematically analogous if one thinks of the electromagnetic field as the photon wave function. The low index medium can be thought of as a potential barrier through which photons can tunnel.
The transmission coefficient for FTIR is very sensitive to the separation between the third medium and the second medium (the function is approximately exponential until the space is almost closed), so this effect has often been used to modulate optical transmission And reflection with a great dynamic distance.