Playing with two rays of light

Abstract

The special theory of relativity is currently included in the curricula of high school in many countries. For this reason, we often organize in-depth lessons on this topic and, in this paper, we summarize a lecture that we have given in some extra activities addressed to last year high school students stimulating pupils "to play" with Lorentz transformations. We found a simple relation that allows us to deduce the rest length of a relativistic train without knowing the relative velocity and without using the lorentz factor and spacetime distortions.