Fibre optic apparatus, late 20th century.
© Science Museum / Science & Society Picture Library
A fibre optic, or light pipe, works on the principle of total internal reflection. When light shines through one end if the fibre, it bounces off the fibre's inside surface until it emerges at the other end - even if this distance is many miles away. Fibre optics can carry signals in the form of pulses of light, just as wires can carry signals as pulses of electricity. The fibres are made from exceptionally pure glas, stretched until it is about 0.5mm in diameter.