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 Bacońs 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.
