This paper reconstructs the development of Mersenne's reflections concerning optics. I argue that Mersenne's optical writings provide crucial insights into Mersenne's Aristotelianism. I reconstruct Mersenne's attempt of explaining the new ideas on light, which were advanced by Kepler, Descartes and Hobbes within Aristotle's natural philosophy. Mersenne explained Kepler's work on light within the Scholastic tradition. In the 1640s, Mersenne was stimulated by the debate concerning Descartes' theory of light, which he accepted only in 1648. Indeed, Mersenne first explained Descartes' law of refraction by means of Hobbes physics. However, Mersenne always wanted to remain faithful to Aristotle.