Whn a ray of light, travelling from an optically denser medium into an optically rarer medium is incident at an angle greater than the critical angle for the two media ,the ray is totally reflected back into the medium , obeying the laws of reflection.This phenomenon is called total internal reflection.
The two conditions for total internal reflection :
1. The ray of light must travel from a denser medium into a rarer medium.
2. The angle of incidence in the denser medium must be greater than the critical angle for that pair of media.