Optical fibres emitting light, with human fingers. These fibres are used for transmitting sound and images in the form of light. Optical fibres 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, since there is little or no absorption of the light being used to transmit the data.