This document discusses refraction of light and total internal reflection. It defines refraction as the deviation of light's path when passing from one medium to another of different density. It explains that light slows down more in an optically denser medium. Total internal reflection occurs when light passes from an optically denser medium to a rarer one at an angle greater than the critical angle, causing the light to reflect back into the denser medium. Examples of total internal reflection applications include optical fibers and mirages.