If the angle of incidence in denser medium is increased further (i > θc), then the incident ray of light will be totally reflected in the denser medium. In this case no refracted ray can be observed. In this situation the separating surface of the two media behaves like a mirror. This phenomenon is called the total internal reflection.
In above fig, the angle of incidence in denser medium ˄P2QNˊ is greater than the critical angle θc of the two media. For this reason the light ray P2Q incidents on separating surface AB and reflects along QR2 following the laws of reflection.
Conditions for total internal reflection:
i) The ray of light must incident from a denser medium at the surface of separation of a denser and a rarer medium.
ii) The angle of incidence in the denser medium must be greater than the critical angle.