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MODEL RELEASED. Fibre optics. Optical fibres being wound onto a spool, monitored by a technician. Optical fibres are made of a flexible glass and have a high refractive index. They are coated with an outer protective layer that has a low refractive index. When light is passed through the fibre in pulses, the beam that emerges from the end of the fibre is completely internally reflected. The light may thus travel along the entire cable with little or no absorption. The pulses of light may carry computer data or telephone conversations, for example. (KEYSTONE/SCIENCE PHOTO LIBRARY/MAXIMILIAN STOCK LTD)