When light is incident on an interface between a more optically dense medium and a less optically dense medium and the angle of incidence exceeds a certain value, called critical angle, all the light is reflected back to the more optically dense medium. This effect is called total internal reflection. In this photograph three beams of white light are totally internally reflected at the two sides of a right angle prism. Note that after two reflections the beams travel in exactly the opposite direction compared to the incident beams. Such an arrangement of reflective surfaces at the right angle to each other is called a corner reflector or retroreflector. Corner reflectors can be found in tail lights on vehicles and bicycles. Corner reflectors were left on the Moon by the Apollo missions as part of the Lunar Laser Ranging Experiment.