1 and n2 are the indices of refraction for each material and 01 and 02 2 represent the incident-beam angle with the normal and the refracted beam angle with the normal, respectively. A second factor that enables light to propagate down the length of an optical fiber is based on the angle of the rays injected into the fiber. For light rays to be reflected, the rays must be injected at incident-beam angles less than what is referred to as the critical angle. Thus, before proceeding fur- ther, let's again digress and examine the role of the critical angle. The Critical Angle The critical angle is critical as it determines why your digitized phone calls, Internet surfing efforts, and other information flowing as light within an optical fiber doesn't leak out of the fiber. Thus, when a beam of light hits the juncture between the core and the cladding, the light beam will be totally refracted back into the core if the angle of the beam with respect to the juncture is greater than some specific value. That value is the criti- cal angle, and this phenomenon is referred to as total internal reflection (TIR); it is called "total" as virtually 100 percent of the beam is reflected. Figure 3.3 A cross-sectional view Jacket of a popular revision to the basic fiber- Strength material optic cable. Buffer Cladding Chapter Three Figure 3.4 The refraction of a beam of light is based on the indices of refraction (n1 and n2) for each material. According to Snell's law of refraction, n sin 01 = n2 sin 02. Figure 3.5 illustrates the flow of light with respect to the critical angle. Note that the farther the beam is from the perpendicular when it strikes the juncture, the more strongly it is bent. We noted from Snell's law that n1 sin 01 = n2 sin 02 Thus, we can use Snell's law to express the critical angle as follows: where 0 is measured from the axis of the core of the fiber. To obtain total internal refraction, the index of refraction of the core (ncore) must be greater than the index of refraction of the cladding = cladding or n cor