This document discusses total internal reflection and its applications. Total internal reflection occurs when light travels from an optically denser medium to a less dense medium at an angle greater than the critical angle, causing the entire light ray to be reflected back into the denser medium. This phenomenon is used in applications like mirages, optical fibers, and cutting diamonds to maximize their brilliance. Optical fibers use total internal reflection to transmit light along their length, and the facet cuts of diamonds are designed to produce total internal reflection and make the diamond sparkle.