. T OTAL I NTERNAL R EFLECTION When light travels from an optically denser medium to a rarer medium at the interface, it is partly reflected back into the same medium and partly refracted to the second medium. This reflection is called the internal reflection. When a ray of light enters from a denser medium to a rarer medium, it bends away from the normal, for example, the ray AO B in Fig.
. . The incident ray AO is partially reflected (O C) and partially transmitted (O B) or refracted, the angle of refraction ( r ) being larger than the angle of incidence ( i ). As the angle of incidence increases, so does the angle of refraction, till for the ray AO , the angle of refraction is / The refracted ray is bent so much away from the normal that it grazes the surface at the interface between the two media.
This is shown by the ray AO D in Fig. . . If the angle of incidence is increased still further (e.g., the ray AO ), refraction is not possible, and the incident ray is totally reflected.