This initial concern is particularly crucial for actual fibers since the diameters of the fibers and the laser beam are very small. The ends of a fiber must be well cleaved, and the fiber itself must be well coupled to the light source, if a significant fraction of the light is to enter the fiber. Directing laser light into an end of a rectangular piece of plastic. As the angle of incidence of the beam is increased, the beam will reflect more often from the inner surfaces in its passage through the plastic. Contrast this behavior with what light does in passing through the sides of the piece of plastic.
Some light is reflected at the surface.
When the refracted beam reaches the back surface, some of the light is internally reflected, and some is refracted, passing through the surface into the air. Directing laser light into a side of a rectangular piece of plastic. The process of reflection and refraction continues as the light beam passes through the plastic. The strength of the beam, however, decreases significantly each time the light makes contact with the surface, since some light is refracted at each successive point of contact. In Figure 10 the initial angle of refraction then becomes the internal angle of incidence at the next point of contact with the inner surface.