You are underwater: Why do you see a circle of light above you?
Have you ever sat at the bottom of a swimming pool and wondered about your water ceiling? Most of the surface is a pale blue sheet, and you can’t see through it, even though the water is crystal clear. But right above you, there is a round, transparent window.
And here’s the cool thing: Through this ring, you can see a fisheye view of not only the sky but also things around the edge of the pool, like trees or people sipping mai tais on the pool deck. This interesting effect is caused by the optical properties of water and has a name: the Snell Window.
You can see this even if you don’t spend much time in the water. Maybe, like me, you enjoy watching spearfishing videos on YouTube. Here’s beautiful example of Snell’s window from YBS Youngbloods channel (link will take you to the important 15 second segment).
One interesting thing to note here: As the diver (Brodie) and cameraman descend, the window appears to remain the same size. So what, you ask? Well, think about it: If you film a window in a house as you back away from it, it will appear to get smaller.
In fact, Snell’s window is becoming bigger—see how divers on the surface fill it up less and less? But unlike windows or anything else on land, angle Size, as perceived by your eye, remains the same as distance increases.
The mystery of the abyss! There are some beautiful physics behind all this, so let’s find out, shall we?
Refraction and Snell’s Law
Since light is an electromagnetic wave, it doesn’t need a medium to “propagate” (unlike sound). That means it can travel through empty space—just like sunlight, luckily for us. Since light travels at 3 x 108 meters per second, the trip from the sun to Earth takes about eight minutes.
But something happens when light enters a transparent medium like our atmosphere: It slows down. Air slows it down by only 0.029 percent, but when light enters water, it loses about 25 percent of its speed. It’s like how you slow down when you run from the beach into the ocean, because water is denser than air.
This speed difference varies with different media and is described by its refractive index (N), is the ratio between the speed of light in a vacuum and the speed in a particular material. The higher the refractive index, the slower light travels in that medium. In air, N = 1.00027. Domestically, N = 1.333. In glass, N = 1.5
But here’s the thing: Changing speed also causes direction of light changes. That’s really what we mean when we say “refraction.” You see it when you look at a straw in a glass of water: The part of the straw that’s underwater doesn’t match up with the part that’s above. Why? The bending of light off the part that’s underwater makes you see it in a place where it doesn’t match up.
