The study aims to identify the participants’ representations of light and its propagation from a primary source through the closed space of a black box and in the absence of multiple diffusion and reflection. The elements of explanation then involve two distinct notions: that light propagates in a straight line; and the necessity for the light to reach the eye in order to be seen. Counter-intuitively enough, in both countries, the percentage of correct answers decreases in a similar way as primary school school children grow up, to become a small minority among students at university (11% in Romania and 16 to 27% in France). Percentage of correct drawings among adults varies between 11% (Romanian future primary school teachers) and 27%. Comparaison with their written explanations reveals an excellent concordance.