From what physical concept is Snell's law derived?
It can be quite easily derived using Fermat's principle that light rays will travel in the shortest path (time-wise). It can also be derived by applying Maxwell's equations with the proper boundary conditions. And according to wikipedia, although I'm not familiar with this derivation:
"Snell's law can be derived using interference of all possible paths of light wave from source to observer—it results in destructive interference everywhere except extrema of phase (where interference is constructive)—which become actual paths."
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