I know that light bends when traveling from one optical medium to another. The "classic" answer to this question is of course because the average velocity of light changes in the process. I've also seen other ways to make it "intuitive" enough to take it as a hard fact, eg. thinking of light as being like a moving train with two poles sticking out laterally so that when it hits an optical surface at an angle, a "torque"(in the sense that its direction of propagation is changed) is experienced by it. As an easy way to visualize the net effect, it is great, but it lacks any theoretical justification. So, is there any theory that would _require_ light to bend while passing from one medium to another and thus describe this phenomenon? I know that the goal of physics is to describe the 'how' about what we observe in nature, make it easier to play with facts, more than ask why, which would be philosophizing, but is there any "abstraction" available for this problem?