This paper examines three explanations of the light refraction proposed by the medieval thinkers Roger Bacon, John Pecham and Erazmus Ciolek Witelo. It is shown, first, that Bacon´s explanation was supported in a metaphysics of the multiplication of causal powers and, second, this explanation arrives to an insurmountable conflict. In the case of Pecham, it is shown that there is a greater commitment with a quantitative law which, unfortunately, leads to an defeated explanation. Finally, the desire of Witelo to introduce cosmological elements in its explanation is valued.