Editorial Feature

Total Internal Reflection - Working Principle and Applications

May 12 2014

Total internal reflection is the reflection of the incident light, which hits a medium boundary at an angle greater than the critical angle, relative to the surface. Total internal reflection takes place only when the following two conditions are satisfied:

The angle of incidence is greater than the critical angle
The ray of light travels from a denser medium to a rarer medium

This optical phenomenon occurs in a number of waves, including sound waves and electromagnetic waves.

Working Principle

When a light wave passes through a boundary between two materials of different refractive indices, a part of the wave will be refracted at the surface of the boundary, and another part will be reflected.

If the angle of incidence of the light wave is greater than the critical angle, i.e. the incidence angle at which the light is refracted and travels along the boundary, then the light wave cannot pass via the boundary. Under this condition, the light is said to be totally reflected back internally.

Frustrated Total Internal Reflection

The propagation of an evanescent wave is an important limitation factor of total internal reflection. Evanescent waves are near-field waves, which causes exponential decay. Under normal conditions of total internal reflection, an evanescent wave does not affect energy conservation.

However, with the introduction of a third medium with high refractive index at less than several wavelengths distance from the boundary between the first and second media, the propagation of an evanescent wave will transfer the energy from the second to the third medium. This process is known as frustrated total internal reflection.

This property can be used for the input/output coupling of an optical resonator.