Think of a ray of light passing through a lens as a car, with constant speed on the asphalt, suddenly meeting a sandy surface. As soon as the first wheel touches the sand, the car will reduce its speed for the greater friction and therefore that side of the machine will proceed more slowly than the other where the wheel is still on the asphalt. The net effect of the unbalance in speed between the wheels is that the machine turns at the transition from asphalt to sand. The same happens to a ray of light passing through the interface between two different materials (air and glass, in the case of the lens), the speed of light is different in media with different refractive indexes, and the ray direction bends at the interface. This simple but fundamental concept underlies all geometrical optics. It can be shown in laboratory using, instead of a bulky optical setup, a small robot built with LEGO® pieces and driven by an intelligent brick LEGO® Mindstorms NXT.
The realization of the software and hardware for this experiment has been the subject of a laboratory at the University of Firenze.