Have you ever sat on the bottom of a swimming pool and pondered your watery ceiling? Most of the surface is a sheet of light blue, and you can’t see through it, even though the water is clear. But right above you, there’s a round window of transparency.
And here’s the awesome thing: Through this ring you get a fish-eye view that shows you not just the sky but also stuff around the side of the pool, like trees or people sipping mai tais on the pool deck. This cool effect is caused by the optical properties of water, and it has a name: Snell’s window.
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