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Why light bends in water: the lifeguard's shortest-time path (Fermat's principle)
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…
Why a rainbow is always a circle 42° around your shadow
Every rainbow is part of a circle centred on the shadow of your head, with its red edge about 42° away. That number comes from inside a single raindrop…