Optical fibres emitting light. 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 boundary. This allows data to be transmitted over long distances with little or no absorption of light.