e Wavelength Determination We noted that the index of refraction can be expressed by the ratio of the speed of light in a vacuum to the speed of light in a medium. Because the frequency of light is the same in any given medium, we can also express the relationship between the index of refraction and wave- length in each medium. Thus, we can express that relationship as follows for two media: = Figure 3.4 illustrates an application of this relation- ship. Specifically, air has a refractive index of 1.0; thus, n1 = 1.0. Let's assume Understanding Optical Fiber Figure 3.5 The critical angle. When the angle is greater than 0c light is When the angle is less than 0c some light totally reflected back into the core. leaves the core. that the second medium has an index of refraction of 1.47; thus, n2=1.47. = Because we previously assumed that the initial beam of light had a wave- length of 550 nm, we can easily compute the wavelength in the second medium. Because n1 = n212, we obtain 1.0 X 550 nm = 1.43 X 12 Thus In addition to the participation of the refractive index, wavelength, and critical angle in the flow of light in a fiber, another term you will fre- quently encounter is numerical aperture. Thus, prior to discussing the flow of light through an optical fiber, let's focus on what the numerical aperture is and how it governs transmission via an optical fiber. Numerical Aperture The numerical aperture (NA) of an optical fiber provides a measurement of the light-gathering capability of a fiber. Mathematically, the NA reflects the difference in the refractive index of the core and cladding as follows: where n represents the index of refraction of the core of the fiber while n cladding represents the index of refraction of the cladding that sur- rounds the core. As the value of NA increases, the optical fiber permits more light to be transmitted through the fiber. Now that we have seen the relationships Chapter Three associated with the indices of refraction of the core and cladding of a fiber, let's determine how lig