Optical fibres emitting light. These fibres are used in high-speed communications networks, transmitting data coded in a beam 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.