So You’re Underwater: Why Do You See That Circle of Light Above You?

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 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. You can see this even if you don't spend much time underwater. Perhaps, like me, you prefer to watch spearfishing videos on YouTube.

Here is a beautiful example of Snell's window from the channel YBS Youngbloods (the link takes you right to the 15-second segment of interest). One curious thing to notice there: As the diver (Brodie) and the camera person descend, the window seems to stay the same size. So what, you ask? Well, think about it: If you filmed a window in your home as you backed away from it, it would appear to get smaller.

In fact, Snell's window is getting bigger —see how the diver on the surface fills less and less of it? But unlike a window or anything else on dry land, its angular size, as perceived by your eye, stays the same as the distance increases. Mysteries of the deep! There's some beautiful physics behind all this, so let's investigate, shall we? Refraction and Snell’s Law Since light is an electromagnetic .