Physics · The quickest path
Why does light bend when it enters water?
What you’ll see
Shine a light into water, and the beam bends at the surface. Why would it do that? Here's a clue. Light moves through water at only three quarters of its speed in air. So suppose it has to get from this point, to that one. The straight line isn't the quickest route. Slide the crossing point along the surface, and watch the total time. It falls, then rises. At the very bottom, the path bends by exactly the angle real light takes. Sine over speed is the same on both sides. Light follows the path of least time.
