Total internal reflection can occur when light moves from a medium of high index of refraction to a medium of low index of refraction. Hence, total internal reflection is complete reflection of light ray, at the boundary of the two media, when the ray moves from higher refractive index to low refractive index.

Huygens considered that light was propagated as waves. Hence, here we consider the incident light not as a ray but as a wave front. Whereas prior to Huygens theory, light was considered as a particle and ray of light was used for representation. Both diagrams are given below for easy understanding.