Light projected through the bottom face of the wedge-shaped slab travels upwards in the same way as light travels through an optical fibre - bouncing at a shallow angle off its front and rear faces. As the light travels upwards and the slab tapers, however, the rays are deflected so that they hit the front surface of the slab at an ever-steeper angle.
Eventually the rays reach the critical angle at which they are no longer reflected but instead exit from the front of the slab. The shallower the angle at which light enters the bottom of the wedge, the further up the screen it travels before exiting. Using two suitably angled projectors, the Wedge can be used to send different images to the viewer's left and right eye, creating a glasses-free 3D effect.