Total internal reflection at the hypotenuse of a right angle prism. When light is incident from a more optically dense medium onto a less optically dense medium (for example, from glass onto air) it refracts (bends) away from the normal to the surface between the media. The angles of incidence and refraction are related through the Snell's law. In this photograph a Helium-Neon laser beam (633 nm) is totally internally reflected at the hypotenuse of a right angle prism. Characteristic laser speckle pattern is apparent. Such an arrangement is often used in optical instruments to deflect light by 90 deg.