Hello forum, please take a look at the following picture: It's a salt solution, with increasing refractive index, as you go down the solution. How can I explain this with Fermat's principle? Let's set the starting point A to the point, where the laser beam penetrates the left wall of the container. The ending point B shall be the intersection of the laser beam with the right container wall. I suppose, that A and B are at same height. Fermat says, that a path between a starting point A and an ending point B needs to be extremal in terms of optical pathlength. Apparently the optical path length of the red arc in the picture is shorter than the linear green path from A to B, because the linear path is in a more refractive medium. But I could also go from A to B on a linear path by pointing the laser perpendicular to the container wall, so that the beam travels along a constant index of refraction, and therefore experiencing no deviation from the linear path. Seemingly Fermat's Principle doesn't give all possible solutions here. In fact, Fermat's principle would forbid the green path, because its not extremal in terms of path length.