Fiber optic cables are bundles of thin, transparent glass threads that transmit messages by light.
The light is transmitted in coded pulses.
A thin layer of glass, called cladding, surrounds each fiber and stops light from escaping.
The cladding reflects all the light back into the fiber so that it bends round with the fiber. This is called total internal reflection.
Single-mode fibers are very narrow and the light bounces very little from side to side. These fibers are suitable for long-distance transmissions.
Aiming light into the narrow core of a single-mode fiber needs the precision of a laser beam.
Multi-mode fibers are wider than single-mode fibers. They accept LED (light emitting diodes) light, so they are cheaper but they are unsuitable for long distances.
The largest cables can carry hundreds of thousands of phone calls or hundreds of television channels.
Underwater fiber optic cables transmit signals under the Atlantic and Pacific Oceans.
Optical fibers have many medical uses, such as in endoscopes. These arc flexible tubes, with a lens on the end, that are inserted into the body to look inside it. Optical fibers are used to measure blood temperature and pressure.