When light travels from a more optically dense medium to 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. No light enters the less dense medium, except a very thin layer near the surface which is referred to as an evanescent wave. This effect is called total internal reflection. In this photograph a diode laser beam (635 nm) is totally internally reflected at the interface between a transparent acrylic slab and air. The acrylic slab is specially formulated to scatter some light and make the beam path visible from the side.