Strands of glass fiber, commonly known as optical fibers, use the principle of total internal reflection to transmit information near the speed of light. The fiber has an inner core of glass with a high refractive index surrounded by a cladding of glass with a lower index. A laser sends light into the end of a strand of fiber. When the light strikes the interface between the core and the cladding, the light is reflected back into the cable, continuing to move down the length of the fiber.

Light or near-infrared radiation can travel kilometers through fibers without significant energy loss. One reason is the total internal reflection. The second reason is that the are made from materials designed to absorb as little as possible of the infrared radiation. A second advantage of optical fibers is that information sent through the fibers is more secure because it doesn’t escape the fiber and thus be accessible to those trying to intercept the information.