The practical propagation of light through an optical fiber may best be explained
using ray theory and Snell’s law. Simply stated, we can say that when light passes from
a medium of higher refractive index (n1) into a medium of lower refractive index (n2),
the refractive ray is bent away from the normal. For instance, a ray traveling in water
and passing into an air region is bent away from the normal to the interface between
the two regions. As the angle of incidence becomes more oblique, the refracted ray is
bent more until finally the refracted energy emerges at an angle of 908 with respect to
the normal and just grazes the surface. Figure 9.24 shows various incident angles of
light entering a fiber. Figure 9.24b illustrates what is called the critical angle, where
the refracted ray just grazes the surface. Figure 9.24c is an example of total internal
reflection. This occurs when the angle of incidence exceeds the critical angle. A glass
fiber, for the effective transmission of light, requires total internal reflection.