Optical fibres. A bundle of optical fibres behind a telephone handset cable. The end of the optical fibres are bright indicating that light is travelling through them. Optical fibres are made of flexible glass fibres coated with a protective coating. The glass has a high refractive index (RI) but the coating has a low RI. This will mean that light emerging from the glass will be refracted so much it will totally internally reflect. The light will thus travel through the cable with little or no absorption. Optical fibres have great value in telecommunications. This is because signals can travel vast distances through optical fibres without needing any boost.