Light has long been used as a long-range signaling medium. While communication by light through open air is still possible using modern technology, it is far more practical in most cases to channel the light signals through a special strand of an optically transparent material called an optical fiber. When packaged in a protective sheath, it is known as a fiber optic cable.
The transmission of light through a “light pipe” was demonstrated as early as 1842 by Daniel Colladon and Jacques Babinet in Paris, using a running stream of water to guide a beam of light. Many modern houses in the United States are equipped with light-pipes19 directing natural sunlight into rooms for illumination, without the use of “skylight” ceiling windows. Modern fiber optic cables apply similar optical principles to very small-diameter fibers of transparent material (usually ultra-pure glass), able to convey optical energy and optically-encoded information.