Hi, While reading about Fermat's principle, and how to use it to derive Snell's law, I came across something I can't find any information about... I had heard about Fermat's principle before; that it said that the path taken by a lightbeam is the path that takes the least time. Now however, I read that it is not just the least time, but sometimes also the longest possible time. The book in question doesn't elaborate further, it just states this fact... When does a lightbeam takes the longest possible path, and why?