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So You’re Underwater: Why Do You See That Circle of Light Above You?
Snell’s window is one of the coolest phenomena in the natural world. You’ve seen it, but do you know what causes it?
For a given light source in the air, we measure the “incident angle” with respect to a vertical line perpendicular to the surface (see the picture below). You can see the blue sky and clouds through Snell's window, but the green color around it in this case is light reflecting off the sand on the seafloor. It turns out that by using light, you can transmit data at a much higher rate than by electrical signals, so it's great for streaming hi-def videos of spearfishing.
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