Full internal reflection occurs when light enters a light medium from a dense medium and falls at an angle greater than the crisis angle with the perpendicular to the difference between the two media. Since the angle of incidence of light is equal to the crisis angle, the reflected ray touches the differential surface of the two mediums , so if light is incident at an angle greater than the crisis angle, it will return to the previous medium without entering the next medium. This process is called full internal reflection because all the rays that fall in this process return through the first reflection.