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

Have you ever sat connected the bottommost of a swimming excavation and pondered your watery ceiling? Most of the aboveground is simply a expanse of airy blue, and you can't spot done it, adjacent though the h2o is clear. But close supra you, there's a circular model of transparency.

And here's the awesome thing: Through this ringing you get a fish-eye presumption that shows you not conscionable the sky, but worldly astir the broadside of the pool, similar trees oregon radical sipping mai tais connected the excavation deck. This chill effect is caused by the optical properties of water, and it has a name: Snell's window.

You tin spot this adjacent if you don't walk overmuch clip underwater. Perhaps, similar me, you similar to ticker spearfishing videos connected YouTube. Here is simply a beautiful illustration of Snell's window from the transmission YBS Youngbloods (the nexus takes you close to the 15-second conception of interest).

One funny happening to announcement there: As the diver (Brodie) and the camera idiosyncratic descend, the model seems to enactment the aforesaid size. So what, you ask? Well, deliberation astir it: If you filmed a model successful your location arsenic you backed distant from it, it would look to get smaller.

In fact, Snell's model is getting bigger—see however the diver connected the aboveground fills little and little of it? But dissimilar a model oregon thing other connected adust land, its angular size, arsenic perceived by your eye, stays the aforesaid arsenic the region increases.

Mysteries of the deep! There's immoderate beauteous physics down each this, truthful let's investigate, shall we?

Refraction and Snell’s Law

Since airy is an electromagnetic wave, it doesn't request a mean to “wave in" (unlike sound). That means it tin question done bare space—as sunlight does, luckily for us. Since airy travels astatine a velocity of 3 x 10⁸ meters per second, this travel from the prima to Earth takes astir 8 minutes.

But thing happens erstwhile the airy enters a transparent mean similar our atmosphere: It slows down. Air slows it by conscionable 0.029 percent, but erstwhile airy enters h2o it loses astir 25 percent of its speed. It's conscionable similar however you dilatory down erstwhile you tally from the formation into the ocean, due to the fact that h2o is denser than air.

This velocity differential varies for antithetic media, and it is described by its scale of refraction (n), which is the ratio of the velocity of airy successful a vacuum to the velocity successful a peculiar material. The higher the scale of refraction, the slower airy travels successful that medium. In air, n = 1.00027. In water, n = 1.333. In glass, n = 1.5

But here's the thing: Changing velocity besides causes the direction of the airy to change. That's really what we mean by “refraction.” You spot it erstwhile you look astatine a straw successful a solid of water: The portion of the straw underwater doesn't lucifer up with the portion above. Why? The bending of airy disconnected the underwater information causes you to spot it determination that it's not.