ively. Then according to Snell's law the incident and re- dium II and all the incident light is reflected back into medium I. This basic behaviour at the interface of media I and II with fracted light rays satisfy Equation 53.1, and in general there is also a reflected ray which propagates in medium I. can be used to guide the propagation of light. The most practical structure for the propagation of light over long (53.1) distances has been the optical fibre, shown in Figure 53.2. The optical fibre is a cylindrical structure with a central region, called the core, and a surrounding external cylinder, called the cladding, Consider now the case where as in Figure 53.1. Then as with the refractive index of the core being higher than that of the the angle Q1 approaches a critical value the refracted ray ap- cladding. If now a light ray is incident at the interface between core proach the extreme angle of and at this point transmission of and cladding at an angle greater than Qcr then the light will propa- gate along the fibre through a series of consecutive total reflections. This description of the propagation of light assumes ideal interface conditions and any departure from them will lead to both reflection and refraction and the light ray will lose energy as it is guided by the fibre. Since the fibre guides light, one of the basic features is the ability to collect light from an optical source. This parameter is known as the numerical aperture of the fibre, NA, and in physical terms is half the apex angle of the light acceptance cone, as shown in Figure 53.3. It is given by Equation 53.3, where An is the difference in refractive index between core and cladding. NA = n sin r (53.3) IT /2 For instance if An = 1% and n, 1 1 5 then NA = 0.212 and the light acceptance cone has an apex angle of 24.5°. The propagation of light in a fibre can be understood at least qualitatively through the concepts of mode and that of the nor- malised frequency. An optical wave propagating by internal reflec- = & cr tion can be represen