What is total internal reflection ?

Consider the following situation. A ray of light passes from a medium of water to that of air. Obviously, the light ray will be refracted at the junction separating the two media. Since it passes from a medium of a higher refractive index to that having a lower refractive index, the refracted light ray bends away from the normal. At a specific angle of incidence , the incident ray of light is refracted in such a way that it passes along the surface of the water.

This particular angle of incidence is called the critical angle. Here the angle of refraction is 90 degrees. So how is the critical angle related to total internal reflection? Any light rays with angles of incidence greater than the critical angle will not cross the water medium at all, will they? They will be “reflected” back into the same medium. This is Total Internal Reflection.