total reflection
In optical transmission in optical waveguides, the light beam is totally reflected between the core glass in which it is guided and the cladding glass.
In optical waveguides, reflections are due to different densities of the two types of glass and to the coupling angle at which the light enters the core glass.
As long as the coupling angle of the light falls below a certain acceptance angle, the light coming from the core glass experiences total internal reflection at the cladding glass. Only when the coupling angle becomes very steep and exceeds a certain value, namely the critical angle, is the light beam no longer reflected back into the cladding glass, but emerges from the cladding glass and is no longer available for optical transmission.