An incident ray traveling in a slower medium strikes a boundary with a faster medium. What happens when its angle of incidence is greater than the critical angle?

The phenomenon that is observed when an incident ray travelling in a slower medium strikes the boundary of a faster medium with an angle greater than the critical angle is known as total internal reflection.

Total internal reflection results in the the ray being reflected within the medium, instead of leaving it after undergoing diffraction. This phenomenon is the working principle of optical fibers.