The subject is Fermat's principle (light takes the path which requires the shortest time) as an example of a general principle which allows an unitary interpretation of different phenomena; a kind of reasoning typical of variational principles is introduced. The first step is a challenge: how to move from a point to another, with the same speed, in the shortest time, after touching a line.
The paths are shown by small rods, marked with small lines so as to show the covered space in the same portion of time: so travelling times become visible. By moving the slide, the fastest path is searched, which coincides with the shortest. A reflected laser ray follows the same path.
Second challenge: which is the quickest path to allow a lifeguard to reach and save a drowning man from the beach? Now speeds are different, the quickest path is no longer the shortest and it is angled according to Snell's law. An optical experiment is repeated with a refraction.
The work has been accomplished and presented by students at the exhibition Scienza Under 18 at the Museo Nazionale della Scienza e della Tecnologia in Milano
(Italy).