When light is incident on a glass-air interface some light refracts (bends) according to the Snell's law while some light is reflected back to glass. When the angle of incidence exceeds a certain value, called critical angle, all the light is reflected, the effect called total internal reflection. Here a right angle glass prism and five beams of light are used to demonstrate the phenomenon. The beams originate at the bottom left side of the photograph and travel at slightly different angles. For the first two beams (counting from left) the angle of incidence on the hypotenuse is below the critical angle. The third beam is incident on the hypotenuse at the critical angle. For the last two beams the angle of incidence on the hypotenuse is above the critical angle.