 5.3 is referred to as a "meridional ray" because it passes through the axis of the fiber. For rays not satisfying this condition, see Senior (1992). Clearly, the picture of total internal reflection assumes an ideal situation without imperfections, discontinuities, or losses. In a real-world fiber, imperfections will introduce light that refracted as well as reflected at the boundary. In arriving at the basic idea of light propaga- tion, it was assumed that somehow the ray has been launched into the fiber. For a ray to be launched into the fiber and propagated it must arrive at the interface between the two media (with different refractive indices) at an angle that is at minimum equal to Ocrit and in general less than that. Figure 5.4 illustrates the geometry for the deriva- tion of the acceptance angle. To satisfy the condition for total internal reflection, the ray arriving at the interface, between the fiber and outside medium, say air, must have an angle of incidence less than Oacc, otherwise the internal angle will not satisfy the condition for total reflection, and the energy of the ray will be lost in the cladding. Consider that a ray with an incident angle less than the Oacc, say Q1, enters the fiber at the interface of the core (n1) and the outside medium, say air (no), and the ray lies in the meridional plane. From Snell's law at the interface we obtain no sin O1 = N1 sin6 O2 (5.4) Chapter 5 Optical Fibers From the right triangle ABC (Figure 5.4), the angle is given by (5.5) where the angle is greater than the critical angle. Substituting equation 5.5 into equation 5.4 we obtain nosing (5.6) In the limit as the incident angle, Q1, approaches Oacci the internal angle approaches the critical angle for total reflection, Ocrit. Then, by introducing the trigonometric relation sin²e + cos20 = 1 into equation 5.4, we obtain (5.7) This equation defines the angle within which the fiber can accept and propagate light and is referred to as the "Numerical Aperture" (NA). (5.8) When the medium with refractive index