Science & Technology

Why Do You Need Goggles to See Underwater?

Our ancient ancestors may have started in the water, but these days, we rely on special equipment to see beneath the depths. We've invented contact lenses and laser technology to perfect our eyesight on land, but underwater, we're still stuck wearing dorky goggles. What is it about water that makes it so hard to see through?

Seeing the Underwater Picture

Whether on land, in water, or even in a giant tub of jello, your eyes work by bending light so that an image registers in your brain. One of those places works better than the others, however. The human eye has evolved to work perfectly on land, but its ability relies on being in the terrestrial air — the second we go underwater, everything becomes fuzzy.

If you recall the days of science class, you might remember how intricately complex the human eye is. The three main parts that help you see clearly are the cornea and the lens, which bend and focus the light to form an image, and the retina, which sends that image to the brain. About two-thirds of the focusing occurs in the cornea and the pillow of aqueous humor (aka water liquid) behind it, with the lens helping out with the final third of the work. And when it comes to the cornea's focusing capabilities, shape really matters: the human cornea is pretty round in comparison to other creatures. So what goes wrong when we take the plunge?

Come on in, the Water's Blind

On land, the cornea and lens independently refract, or bend, light so that a collective image forms for your brain to interpret. But underwater, your cornea isn't able to work in the same way. When submerged, the cornea is too similar in consistency to water, meaning there isn't enough of a difference between the two for your light-focusing system to work as it should. Instead, the lens has to jump into action and make up for the cornea's loss of focusing power. The result? An image that's sort of there, but often too blurry to be of much help.

Animals that live their entire lives underwater have vastly different visual systems than our own. The eyes of fish, for example, rely solely on their lens. But while the human lens is biconvex (curved on both sides, like the lens of a telescope), a fish's lens is spherical, which better helps it bend light underwater. Because they don't rely on the cornea (or aqueous humor) to see, it allows their underwater vision to trump ours — though take a fish out of the water, and their vision is practically useless. But many creatures can see clearly both below water and above. Seals and penguins both have relatively flat corneas, which don't lose as much power as ours do when they're underwater.

There are documented groups of people who can see underwater better than most, however. The Thai Moken people, also known as the "Sea Nomads," have lived as a coastal culture for thousands of years and demonstrate a heightened ability to see underwater. A 2003 study of Thai Moken children suggested they achieved this feat by shrinking their pupils to an extreme degree. Still, if you don't spend your days diving for sea cucumbers, you're unlikely to achieve that visual adaptation.

Air to the Rescue

But if you want to explore below the surface, how do you solve the problem of having land-specific eyes? Easy: goggles. By adding a layer of air between the water and your cornea, your visual system can go back to working properly. That simple pocket of air lets the cornea and its pillow of watery liquid go back to bending light and focusing an image, just like it does on land. That's humanity's real achievement after all: We might not have the strongest muscles or the sharpest eyes in the animal kingdom, but we can invent things that get us there all the same.

For more on how your vision operates, check out David Macaulay's fun explainer, "Eye: How It Works." We handpick reading recommendations we think you may like. If you choose to make a purchase, Curiosity will get a share of the sale.

Ask a Scientist: Underwater Eyes

Written by Ashleigh Papp July 10, 2018

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