Retroreflector. Illustration showing a light source (gold cylinders) firing photons (blue spheres, red spheres) at a corner cube reflector (centre left). The retroreflector always directs light back to its source, regardless of the angle at which it arrived. The photons are reflected in a way analogous to balls bouncing from a surface. The corner cube is made up of three reflective surfaces at 90 degrees to each other. Each surface bounces the balls back in one axis. When the balls bounce off all three surfaces, they travel back parallel to their original course. The same principle occurs with light, and retroreflectors like this are used in road signs, emergency equipment and surveying tools.