Why light bends in water: the lifeguard's shortest-time path (Fermat's principle)
30seconds
A lifeguard runs faster than she swims, so the quickest route to a swimmer is not a straight line. It bends at the shoreline, and the bend follows one tidy rule.
Light obeys the same rule when it enters water or glass. That rule is Snell's law, and Fermat's principle explains it: light takes the path that needs the least time.
Drag the entry point yourself, watch the time curve, and find the bend that wins.
Deep dive · 4 min
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