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Fibre optics. Bundle of optical fibres conducting light. These fibres are made of flexible glass that has a high refractive index. Each fibre is then coated with an outer protective layer that has a low refractive index. The light beams that emerge from the end of each fibre have been completely internally reflected. The light may thus travel along the entire cable with little or no absorption. Bundles of optical fibres may be used to carry images, or high volumes of data such as telephone conversations. (KEYSTONE/SCIENCE PHOTO LIBRARY/STEVE HORRELL)