Have you ever sat at the bottom of a swimming pool and thought about the water ceiling? Most of the surface is a light blue sheet and you can’t see through it, even though the water is clear. But right above you is a transparent round window.
And the best part is that through this ring you get a fisheye view that not only shows the sky, but also what’s around the pool, like trees or people drinking mai tais on the pool deck. This interesting effect is due to the optical properties of water and has a name: Snell’s window.
You can see this even if you don’t spend much time underwater. Maybe, like me, you prefer to watch underwater fishing videos on YouTube. Here’s a Beautiful example of Snell’s window from the YBS Youngbloods channel (the link takes you directly to the 15-second segment of interest).
One curious thing to notice here: as the diver (Brodie) and the cameraman descend, the window appears to remain the same size. So what? you might ask. Well, think about it: if you were to film a window in your house as you moved away from it, it would appear to get smaller.
In fact, Snell’s window is getting bigger and bigger. bigger—See how the diver on the surface fills less and less? But unlike a window or anything else on dry land, his angular The size, as perceived by the eye, remains the same as the distance increases.
Mysteries of the deep! There’s some beautiful physics behind all this, so let’s dig in, shall we?
Refraction and Snell’s law
Since light is an electromagnetic wave, it doesn’t need a medium to “transmit” it (unlike sound). That means it can travel through empty space, just like sunlight does, luckily for us. Since light travels at a speed of 3 x 108 meters per second, this journey from the Sun to the 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 it enters water it loses about 25 percent of its speed. It’s like how you slow down when you run from the beach to the ocean, because water is denser than air.
This speed difference varies for different media and is described by their refractive index (north), which is the ratio of the speed of light in a vacuum to the speed in a given material. The higher the refractive index, the slower light travels in that medium. In air, north = 1.00027. In water, north = 1,333. In glass, north = 1.5
But here’s the thing: changing speed also causes address Light changes. That’s what we mean by “refraction.” We see this when we look at a straw in a glass of water: the part of the straw that’s under the water doesn’t match the part that’s above it. Why? The bending of the light from the part under the water causes us to see it in a place where it isn’t.