The complete reflection of a light ray as it strikes the interface between the medium in which it is traveling and a medium with a lower refractive index, or index of refraction (IOR) at an angle greater than the critical angle, which is measured from the perpendicular at the point of reflection. Depending on the specific nature of the glass optical fiber (GOF) and its manner of construction, oblique light rays striking the interface between the core and cladding variously are reflected or refracted back into the core, which is the primary light-conducting medium. If the optical fiber is a step-index fiber, and the angle of incidence is greater than the critical angle, the light rays are reflected back into the core. If the angle is less than the critical angle, the light rays penetrate the core/cladding interface, where they are lost. If the fiber is a graded-index fiber, the light rays also reflect off the core/cladding interface if the angle of incidence is greater than the critical angle. As the angle decreases below the critical angle, the light rays enter the cladding, where they gradually gain velocity and bend, or refract, back into the core. The lesser the angle, the greater the penetration and the greater the associated increases in velocity and refraction. If the angle of the incident light ray is too severe, the light ray will penetrate the core/cladding interface and be lost in the cladding of either type of cable.Total internal reflection essentially confines the optical signal to the core conducting material, thereby maintaining signal strength over a distance.Total internal reflection is the fundamental principle that makes fiber optic transmission possible. See also angle of incidence, cladding, core, critical angle, graded-index fiber, IOR, reflection, refraction, and step-index fiber.