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If you’ve been brave enough to open your eyes underwater, you might have noticed that everything is blurry. But fish have no trouble finding their way beneath the waves. So why can’t we see as clearly below as we do above?

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[♪ INTRO].

Even if you have 20/20 vision, you might have noticed that when you go swimming, everything underwater is just… a blur. But fish and other underwater animals seem to have no trouble seeing in their natural environment.

So why can’t we humans see underwater? Are our eyes just that bad? Not really -- they’re just adapted to focusing in air.  Our vision depends on light bending, or refracting, when it enters our eyes.  Typically, when light travels from a less dense to a more dense material, the light slows down -- in most cases, anyway.

And when it slows down, it bends. We use a measure called refractive index to indicate how slowly light moves in a material. Denser materials tend to have higher refractive indices.

As light passes from one material to another, how much it bends depends on the change in refractive index, and the angle the light hits at. Think of a spoon in a glass of water. It looks bent because air and water have different refractive indices.

For vision, incoming light first meets the outer part of our eyes, called the cornea. Its dense material causes the light to refract. That, and the curved shape of the cornea, causes the light to bend towards the center of the eye.

After the cornea, the light eventually reaches the lens, which is even denser, further bending the light. All of this works together to create a little focused image that lands right on the back of your eyeball. But water is quite a bit denser than air, and it actually has almost exactly the same refractive index as the human cornea.  This means that when light travels through the water into your cornea, it barely bends at all.

There’s none of the sudden slowing down that usually causes it to refract. Since everything inside the eyeball stays the same, the light will still bend a bit once it hits the lens -- but underwater the human eye loses over half of its light bending power. While focused light hits the back of the eye all in one spot, unfocused light is all spread out, making it hard for the brain to interpret what it’s seeing.

Fortunately, there’s a simple solution. All you need is a little pocket of air in front of your eyes to get that bending power back, so wearing goggles will let you clearly see underwater. Incidentally, when the light travels from the water into your goggles, it will bend a bit thanks to the change in refractive index -- but that just works like a magnifying glass, making things look a little bit closer than they actually are.  So how do fish get around without goggles?

Just like we’re adapted to seeing in air, they’re adapted to seeing in water. Rather than their corneas, fish do basically all their focusing with their lens, which has an even higher refractive index than a human lens. Their lenses also tend to be more spherical, which maximizes their bending abilities.  Of course, there’s a twist here -- the first fish to venture out of the water hundreds of millions of years ago probably couldn’t see too well in air!  They would have had the strong focusing lenses of underwater creatures, and then on top of that, in air, their corneas would kick in and provide even more focusing.

And too much focus is just as blurry as not enough.  So while humans have bad underwater vision, that doesn’t mean there’s anything wrong with us -- in a way, we’ve fixed a problem that arose hundreds of millions of years ago. Thanks for asking! And thanks to our patrons for helping us bring you the answer.

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