I DO NOT like this least time hocus pocus. I prefer the idea of causality. I just CANNOT stomach this idea. Here are my arguments (italicized text) almost verbatim from my notes against what I read (in bold). Someone please explain to me the whys and hows. Arguments against Fermat: "Given a source point A and receiving point B, light takes path of least time to get there" <-----A B If the light ray is sent as in my case, it will never reach B. In such a case a path of least time does not even exist. But if I simply took Snell's law/ angle of incidence = angle of reflection law, I would be able to say point by point where it is going to go. I find that simpler to think of. Correction/ better wording of Fermat's theorem that was made a little later: "light takes such a path such that the paths infinitely close to it take nearly the same time" Light takes such a path? A path to go where? How do you know WHERE the light is going? How do you know it's final destination? Using causality, there was no confusion. At every instant I could calculate WHERE the light was heading. Seems to me you have already pre-determined where it ought to go. I don't get that at all. Why the heck would it want to go to a point B? What's so special about it? Just because you are sitting there, or you WANT light rays to reach there doesn't make it any more special. Only if some light is HEADING your way will it come to you. I can do sums in the Calculus of variations, if you need that to explain to me what's going on. I did the chapter in Boas before I started this stuff. If you don't understand what I've written above and think that I am not clear with what I am trying to say I don't understand, here is an extract my Feynman lectures, following which I have presented the same argument in different words. Feynman is explaining the mirage caused in deserts and hot roads. [Chapter 26, Volume I] ....Why[is the mirage caused]? The air is very hot above the road but it is cooler up higher. Hotter air is more expanded than cooler air and is thinner and this decreases the speed of light less. That is to say, light goes faster in the hot region than in the cool region. Therefore, instead of light coming down in the straight forward way, it also has a least-time path by where it goes faster for a while in order to save time. So, it can go in a curve... 1. First of all, he seems to be fixing yourself as a point that light rays must come to. Why should that be so? Why should the light WANT to come somewhere? If I had a point source, I could decide to send light only in one direction, away from you. Instead of using any least time thing, I could set up a mirror in the direction I am sending the light rays towards and thereby make them reach you. Here, the light didn't WANT to come to you. I made it do so. 2. How do you know a path of least time exists in the first place? Using what I said above, if I didn't have a mirror in the above case, it wouldn't even head in your direction. In such a case a path of least time wouldn't even exist.