Total Reflection and Light Guides
If a wave passes the boundary between two media of different optical densities, it does not continue straight ahead, it will be reflected and refracted. In the case that a wave coming from an optically denser medium (e. g. glass) into an optically thinner medium (e.g. air), there is a certain limiting angle for a total reflection at the boundary where total reflection is observed. When a wave impinges on the separating boundary at an angle smaller than the limiting angle, it becomes reflected and remains completely in the optically denser medium. This can be seen both for light waves and for neutron waves as well, and is described by the Snellius law (see graphic and the formula in the box below). The limiting angle for total reflection is determined by the ratio of the optical densities of the two media. For the transition from glass to air, this ratio is 3:2 and the corresponding limiting angle for the total reflection is 42°.
A well-known example for a technical application of the total reflection of light is information transmission by light signals in very thin glass fibers (light guides) as depicted below.
The total reflection enables here a signal transport in very thin, flexible and transparent fibers with lengths of up to several kilometers, which connect electronic devices and, like electric wires, allow a very quick data interchange.
Texts: Th. Wilpert , Ch. Schulz
