Total internal reflection in an optical fibre, illustration. Total internal reflection occurs when light is incident on an interface between a more optically dense medium (for example, glass) and a less optically dense medium (air) and the angle of incidence exceeds a certain value, called the critical angle. All the light is then reflected back to the more optically dense medium and no light enters the less dense medium. Optical fibres operate using this principle. An optical fibre is a flexible, transparent glass or plastic fibre through which information is transmitted over long distances, coded in light beams. This technology is widely used in communications.