Fiber Optic transmission of light depends on preventing light from escaping from the fiber. When a beam of light encounters a boundary between two transparent substances, some of the light is normally reflected, while the rest passes into the new substance. The angle at which the light strikes the boundary decides how much light is reflected and how much is absorbed. Fiber optics makes use of special conditions, under which all the light encountering the surface between two materials is reflected, to reduce loss.
A principle called total internal reflection allows optical fibers to retain the light they carry. When light passes from a dense substance into a less dense substance, there is an angle, called the critical angle, beyond which 100 percent of the light is reflected from the surface between substances. Total internal reflection occurs when light strikes the boundary between substances at an angle greater than the critical angle. An optical-fiber core is clad (coated) by a lower density glass layer. Light traveling inside the core of an optical fiber strikes the outside surface at an angle of incidence greater than the critical angle so that all the light is reflected toward the inside of the fiber without loss. As long as the fiber is not curved too sharply, light traveling inside cannot strike the outer surface at less than the critical angle. Thus, light can be transmitted over long distances by being reflected inward thousands of times with no loss .
Optical communications date back to the 1790's with French engineer Claude Chappe's "optical telegraph". Chappe's system involved a series of semaphores mounted on towers, where human operators relayed messages from one tower to the next. Of course it was superior to hand-carried messages, but by the mid 1800's it had been replaced by the electric telegraph.
In 1880, Alexander Graham Bell developed and patented an optical telephone system, the Photophone, but his earlier invention, the telephone, proved to be far more practical.