An optical fiber is a long, thin strand of clear material. Its shape is usually similar to a cylinder. In the center, it has a "core". Around the core is a layer called the "cladding". The core and cladding are made of different kinds of glass or plastic, so that light travels slower in the core than it does in the cladding. If the light in the core hits the edge of the cladding at a shallow angle, it bounces off. Light can travel inside the core and bounce off of the cladding. No light escapes until it comes to the end of the fiber, unless the fiber is bent sharply or stretched.