Optical fibres. These fibres are used for transmitting sound and images in the form of light. They are usually made from flexible glass that has a high refractive index. Each fibre consists of a core with an outer protective layer (cladding) that has a lower refractive index than the core. Light travelling through the fibre is contained within the core because of total internal reflection at the core/cladding boundary. This allows data to be transmitted over long distances with little or no absorption of light.