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Ok, OK, but how do you explain this to a student when SHE asks:
"When a beam of light enters the surface of water at an angle of
25 degrees with the normal why doesn't it continue at the same
angle after it enters the water? Does Fermat's principle
explain why the wavefront must always perpendicular to the direction
of travel?
(Where wavefronts have learned that they must ALWAYS be
perpendicular to the direction of wave propagation)