Let us consider a ray of light passing from glass to air, that is from a denser medium to a rarer medium. The figure shows that for a small angle of incidence, major part of the incident light is refracted and a small portion is reflected. The refracted ray bends away from the normal after the refraction. That is, the angle of refraction r is greater than the angle of incidence i. Now if we increase the angle of incidence, the angle of refraction also increases and for a certain angle of incidence (say ic), the refracted ray grazes over the surface of separation and the angle of refraction will be 90o. ic is referred to as the critical angle. If we further increase angle of incidence, the light ray instead of getting refracted bounces back into the same medium obeying the laws of reflection. This is known as total internal reflection.
is that angle of incidence for which a ray of light while moving from a denser to a rarer medium just grazes over the surface of separation of the two media (that is, angle of refraction = 90o).
If the angle of incidence of a ray of light traveling from a denser medium to a rarer medium is greater than the critical angle for the two media, then the ray is reflected into the denser medium and this phenomenon is described as total internal reflection.
