Total internal reflection. Critical angle
When light passes from one medium to a more optically dense medium there will always be both reflection and refraction for all angles of incidence (Fig. 23.11). But this is not always so when light passes from one medium to a less optically dense medium.
Suppose we consider a ray passing from glass to air. Starting with a small angle of incidence (Fig. 23.12 (a)), we get a weak internally reflected ray and a strong refracted ray.
As the angle of incidence increases the angle of refraction also increases, and at the same time the intensity of the reflected ray gets stronger and that of the refracted ray weaker.
Finally, at a certain critical angle of incidence c, the angle of refraction becomes 90° (Fig. 23.12 (b)). Since it is impossible to have an angle of refraction greater than 90″, it follows that for all angles of incidence greater than the critical angle c the incident light undergoes what we describe as total internal reflection (Fig. 23.12 (c)). See also optical fibres, page 272.