Light striking the interface between two substances can either reflect off of the substance it encounters or enter it, with the difference depending on the nature of the substances and the angle at which the incident light ray strikes the interface. A glass optical fiber (GOF) comprises an inner core of glass of a given refractive index, or index of refraction (IOR), surrounded by one or more layers of cladding of lower refractive index.The critical angle is measured from the normal, which is at 90 degrees from (i.e., perpendicular to) the surface of the core/cladding interface, or boundary. If, as illustrated in Figure C-6, the incident light rays strike the interface at an angle greater than the critical angle, they reflect off the interface, with the angle of reflection being the same as the angle of incidence.The light rays glance off of the interface, so to speak. If, on the other hand, the incident light rays strike the boundary at an angle less than the critical angle, they enter the cladding, where they either are lost or refracted back into the core, depending on the type of fiber and the angle of incidence. See also angle of acceptance, angle of incidence, cladding, cone of acceptance, core, GOF, graded-index fiber, IOR, numerical aperture, step-index fiber, and total internal reflection.