Strips of gelatin dessert and a laser pointer demonstrate total internal reflection.
The boundary surface between the gelatin walls and air acts like a mirrored surface because the gelatin has a higher index of refraction, or tendency to bend the path of light. If light should strike the sidewalls above what’s called the critical angle, light will pass through the sidewalls. Below the critical angle, light is reflected as if it were bouncing off a mirrored surface. The critical angle for internal reflection can be observed by adjusting the position of the laser. More efficiently than gelatin, pure glass used in the manufacture of fiber optic cable allows light to pass through undisturbed for long distances. Many cable TV, telephone, advertising signboard, and data transmission systems use fiber optic cables.