The Nature and Propagation of LightTo understand how light propagates through optical fiber, it is important to understand the nature of light and how it propagates. The nature of lightScientists were confused about what light really is. Three main theories have been developed to understand what light is. 1- Light as rays" In the classical physics that many of us learned in school, light consisted of "rays" that could be reflected and refracted through mirrors and prisms, etc. good description as far as it goes, but can't explain many of the phenomena that we use in optical communications" (Dutton, 1998, p. 15). Dutton (1998) illustrates that this theory cannot clarify the phenomena of diffraction or interference in light. 2- Light as photons"In many contexts light behaves as if it were made up of tiny particles called "photons". There are numerous phenomena that the wave model of light cannot explain. The best known of these is the “photoelectric effect” (Dutton, 1998, p. 15).3- Light as electromagnetic waves"In the context of communications optics, in most cases you will find that the best way to think of light is to think of it as an electromagnetic wave. In this respect it is no different from a radio wave, except that the wavelength is much shorter" (Dutton, 1998, p. 15). According to Dutton (1998), the electromagnetic wave is consisting of an electric field and a magnetic field, as shown in figure 1. Figure 1: The electric field and the magnetic field Principles of light propagation According to Brunel University (2007), light is the visible light that Its light wavelength varies from approximately 400 to 700 nm." However, as will be shown, the propagation characteristics of optical fibers dictate… middle of the paper… the core index is uniform throughout and undergoes an abrupt or step change at the core cladding boundary. The light rays propagating through the fiber are in the form of meridional rays which cross the fiber axis during each reflection at the core cladding boundary and propagate in a zigzag manner as shown in the figure. 3a". (Arumugam, 2001, p.851).• The multimode graded index fiber"In graded index fiber, the refractive index of the core is made to vary parabolically such that the maximum value of the index of refraction is at the center of the nucleus. The light rays propagating through it are in the form of oblique or helical rays which never cross the fiber axis and propagate around the fiber axis in a helical (or) spiral manner as shown in figure. 3b". (Arumugam, 2001, p.851).Figure 3: Different types of optical fibers