Why can’t we see under water? And how come penguins can?

One of my most indelible early memories is of participating in a near-drowning incident when I was 8 years old. This was in a lake, near the shore, in water less than three feet deep. A life jacket somehow ensnared my legs, suspending them near the surface and trapping my head under water. Before losing consciousness I saw spidery patterns of light rippling over the sand a few inches from my face, an image that lodged itself with vivid permanence in the inner recesses of my brain.

Or so I have always thought. Last night at a family dinner my dad disputed the accuracy of this visual memory. The human eye is incapable of focusing properly under water, he said, because the refractive index of water is so different from that of air. This led to a prolonged argument, my position fueled by better wine than science: eyes must be able to see under water because a) I remember doing so, and b) what about pearl divers?, and c) what about amphibious creatures like seals and otters?

As a general rule, all worthwhile dinner arguments can be resolved by simple empirical tests after dessert. We sealed an eye chart inside a ziploc bag and put it at the bottom of a large tub of water. I immersed my head in the tub and opened my eyes, pen poised over a notepad, but was unable to discern even the largest of the letters, despite still having my glasses on.

This is probably old news to most people. I blame my ignorance of the phenomenon on only rarely venturing into the water without proper safety gear (goggles, flippers, scuba tank, etc.).

Why can’t our eyes focus under water?

The answer is simple and satisfying.

When light travels from one medium (e.g. air) to another (e.g. water) it is refracted, that is, its path is bent. Although the precise mechanistic details are beyond me, this bending results from the fact that light travels at different speeds in different media. The “refractive index” of a substance refers to how much slower light travels in it compared to in a vacuum, and it increases (roughly) with density. For instance, the refractive index of air is approximately 1, because it slows light down very little. The refractive index of water is 1.333, because it slows light down to around 75% of its speed in a vacuum. The greater the difference in refractive index between two substances, the more light bends when it moves from one to the other. This is why a pencil looks crooked when you poke it into a glass of water. If you could poke a pencil into a diamond (refractive index: 2.42) it would look crookeder still.

Why a pencil looks bent under water. Source: Wikipedia commons.

Our eyes, and those of most other terrestrial vertebrates, exploit this effect by having bulging, rounded corneas and a layer of liquid (the aqueous humour) in front of the pupil. Remember that the whole point of an eye is to bend incoming light to form a tiny image on the retina. Because their refractive index differs from that of air, the cornea and aqueous humour bend light that enters the eye, pre-focusing an image before it reaches the lens.  This greatly increases an eye’s ability to focus – its optical power – because the light can be bent first at the surface of the eye, and again at the lens. In humans the cornea accounts for about two thirds of the eye’s optical power, and the lens accounts for the remaining third.

With this in mind, it is easy to see why our eyes are so poorly adapted to seeing in water. The refractive indices of water and the cornea are so similar that light is hardly bent at all when it enters the eye.  It is bent only by the lens, so that the image is not focused on the retina, but somewhere behind the retina. The effect is like that of a projector positioned too close to the screen.

Emmetropia (normal vision) in air and hyperopia (farsightedness) in water. When the eye is in water light is not bent by the cornea. From Hanke et al. (2009).

Out of the sea and back again

Terrestrial vertebrates evolved from fish, and their eyes would at first have been similar to those of modern fish: adapted to seeing under water, with a hard, spherical lens solely responsible for focusing incoming light. Moving from the sea to land created an extra point of refraction (at the air/cornea interface), which means that our ancestors probably suffered from blurry, myopic vision for millions of years. In time, though, the refractive ability of the cornea opened new avenues for evolution, and turned the handicap into an advantage. Relieved of some of the burden of refracting incoming light, our lenses were freed up to become softer and less dense, more flexible, capable of changing shape to fine-tune their focal distance. That is why the eyes of some modern terrestrial vertebrates – in particular, predatory birds such as hawks and owls – are probably the finest that ever saw.

But what about those animals that, having made the transition from sea to land, returned once more to semi-aquatic lives? How are seals, otters, penguins and puffins able to focus both above and below water?

Puffin on Skomer Island, Wales, in 2011.

In many cases they have evolved modified corneas with reduced refractive power. Penguins, puffins, seals and albatrosses, for instance, have relatively flat corneas and spherical lenses. This type of eye suffers relatively little loss of power under water because the cornea wasn’t contributing much refraction to begin with. Presumably the compromise leaves the animals with mediocre vision in both air and water.

Some diving birds, such as sea-dippers, cormorants and mergansers, solve the problem in a different way. They have strongly curved corneas, but also super-deformable lenses. When submerged they radically distort the shape of their lenses to compensate for their aquatically useless corneas.

Others seem to possess no adaptations whatsoever. Crocodiles, notably, have good eyesight on land but pathetically blurry vision underwater.

And on the 5th day…

There was a rare fish called Cassoorwa, which hath in each eye two sights, and as it swimmeth it beareth the lower sights within the water, and the other above…

- Harcourt 1608, cited in Baughman 1947.

There are, of course, many species of fish that spend parts of their lives out of water. Some intertidal fish have flattened corneas, like seals and puffins. Others have evolved strikingly different solutions to the problems of amphibious vision. For instance, Alticus kirkii, a species of combtooth blenny, has an extra membrane behind the cornea that it can use to create an additional eye chamber, shifting the focal point when it moves between air and water.

My favourite, though, is that of four-eyed fish (genus Anableps). Four-eyed fish have eyes that are divided horizontally, with the lower half adapted to seeing in water and the upper adapted to seeing in air. The pupils are hourglass-shaped, and the two halves of the retina are sensitive to slightly different wavelengths of light.

With tedious predictability, this has been eagerly eized upon by creationists as yet another example of irreducible complexity and further proof of the Lord God’s creative ingenuity.

That seems like a good note on which to end a post about myopia.

The four-eyed fish. Source: ryanphotographic.com.

ADDENDUM: What about the pearl diver thing?

I had forgotten about this question until Piers Fletcher reminded me in the comments section. It turns out that some human populations do have surprisingly good underwater vision; notably, several groups of seagoing nomads in Southeast Asia (usually called “sea-gypsies”, which seems almost definitely politically incorrect). Researchers have compared the underwater spatial resolution of children from some of these tribes to that of European children, and the sea-children fare over twice as well.

There is a physiological (not, so far as is known, genetic) adaptation behind their ability to see under water. The sea-gypsies constrict their pupils, which sharpens their vision. Surprisingly, none of the papers I read explained the mechanism behind this, but I suppose it’s probably that constricting the pupil limits the angle at which light rays can hit the lens, reducing the refractive work that the lens has to do.

One study (Gislen et al. 2006) found that European children began unconsciously to constrict their pupils as well after about a dozen underwater training sessions. So the ability appears to be something that can be acquired fairly easily – although Gislen et al. only used 4 European children in their study, so we should be cautious about drawing any strong conclusions just yet.

Incidentally, the reason why you can see clearly with goggles or a mask is because they reintroduce the air/cornea interface.

 

Reading

Baughman, JL. 1947. An early mention of Anableps. Copeia, 3, 200.

Hanke et al. 2009. Basic mechanisms in pinniped vision. Exp Brain Res 199, 299-311.

Gislen, A & L. Gislen. 2004. On the optical theory of underwater vision in humans. Opt. Soc. Am. A 21(11), 2061-4.

Gislen et al. 2003. Superior Underwater Vision in a Human Population of Sea Gypsies. Current Biology 13(10), 833-836.

Gislen et al. 2006. Visual training improves underwater vision in children. Vision Research 46, 3443-3450.

Katzir, G., & HC Howland. 2003. Corneal power and underwater accommodation in great cormorants. J Exp Biol 206, 833-841.

Sayer, MDJ. 2005. Adaptations of amphibious fish for surviving life out of water. Fish and Fisheries 6, 186-211.

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18 Comments

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18 responses to “Why can’t we see under water? And how come penguins can?

  1. I too did not know that. The explanation as to why we cant focus under water is very good. But I have a couple of questions.
    1. Since, underwater, the image gets focused behind our retina and is similar to hypermetropia, then does that mean looking at far away objects will not be a problem underwater?
    2. You also mention when talking about fish that came to water that “Relieved of some of the burden of refracting incoming light, our lenses were freed up to become softer and more flexible, capable of changing shape to fine-tune their focal distance.” I am wondering how the original fish looked at near and far away objects without the ability to change the focal length of their eyes? or did they not have that ability at all?

    • Thanks for your comment and questions! To answer them in order:

      1. I think you’re right that looking at distant objects under water should be less difficult than looking at close objects. Since the light rays from close objects are diverging when they enter the eye, you need more refractive power to focus them on the retina. If the object you’re looking at is far away the rays will be nearly parallel, and it should be easier for the lens to focus them on its own. Having said that, I don’t actually know whether the lens is capable of adjusting enough to focus perfectly on distant objects under water.

      2. Another great question. Rather than changing the shape of their lens, fish move the lens itself towards or away from the retina, just as you adjust the focus on a camera by moving the lens relative to the sensor.

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  3. So, vision like matter is relative!
    Delightful article!

  4. Piers Fletcher

    Very nice article. Did you ever figure out the answer to this bit:
    “b) what about pearl divers?”
    ?

    • Thanks for reminding me, I did look into this but forgot to write about it. Apparently some humans have adapted to life under water to a surprising degree. There are a number of Southeast populations (known as “sea gypsies”) that spend a huge amount of their time in the ocean – the Moken and Bajau peoples, for example. Studies have shown that children from these groups can tightly constrict their pupils under water, which gives them relatively good vision (about twice as good as the European children in the study). EDIT – I added an addendum to the original post and some extra references.

      • Piers Fletcher

        Fascinating – thanks. On this bit …
        “none of the papers I read explained the mechanism behind this, but I suppose it’s probably that constricting the pupil limits the angle at which light rays can hit the lens, reducing the refractive work that the lens has to do”
        … I would have guessed that the mechanism at work here is the one deployed by photographers when they want to maximise depth of field (ie to have both near and distant objects in focus). The smaller the aperture of your lens, the greater the depth of field – and constriction of the pupil is the same thing as stopping your camera lens down to a small aperture. Or maybe I’m misunderstanding your point.

  5. I think that is absolutely the right analogy, but I have to confess to not having though much about why, exactly, narrowing the aperture increases depth of field – unless it is simply because it restricts the angle at which incoming light can hit the lens.

  6. Piers Fletcher

    There’s an attempt to explain the optics here: http://www.cs.mtu.edu/~shene/DigiCam/User-Guide/950/depth-of-field.html. Don’t know how good it is, mind.

    When I was a kid I used to make pinhole cameras using a shoebox, and if you made the hole (aperture) small enough you didn’t need a lens at all, because EVERYTHING was in focus. So I reckon that’s the effect that’s being used here.

    Sorry, I’ll stop bugging you now.

    • Not bugging me at all, I’m glad someone else finds this stuff interesting! That article does a good job of explaining the optics, I think.

      My dad and I used to make pinhole cameras when I was a kid as well. You must be right that the same effect is in play. I think there are a number of living species with pinhole camera-like eyes (the nautlius and giant clam come to mind), but of course, that’s another post in itself…

  7. Hi.
    I am short sighted (about -5.5). When I swim underwater with goggles, I am convinced that I see better. The goggles complicate things, because there is an air space of about ½ in. between the front of the eye and the water. So I am not sure how you could calculate the effect of the air/water combination. (Would someone with perfect eyesight, looking into a glass-sided pool, see better with optical correction?) I wonder if the pressure on the eye makes a difference?
    The reason I am interested is that I have been tempted to buy optically corrected swimming goggles, but I suspect they would only be of use if I swam with my head out of the water. What do you think?

    • I think I have worked this one out for myself.
      My goggles have slightly curved lenses. (Convex at the interface with the water.) The clear plastic is of even thickness and has negligible optical effect in itself, but it does create a curved interface between water and air and thus acts as a negative lens by refracting light rays away from the normal as they pass into the air space behind the goggles.
      I have not yet had the opportunity to examine different makes of goggles, but I suspect that many will have curved lenses because they will be stronger under pressure.
      So I conclude that goggles with curved lenses will give some correction for shortsighted users below the water surface. They will make no difference above the surface. The amount of correction will depend on the degree of curvature of the lenses.

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  10. Pavel

    I believe that your test was not sufficient to disporve what you saw.

    1. If you are myopic, your glasses probably made your vision worse underwater.

    2. Young children too have lenses that are very flexible, perhaps you were still able to focus well underwater, (especially if you are strongly myopic)

  11. Zak

    I’m not sure this is entirely true, I’ve been swimming a few hundred times. I open my eyes underwater and have been able to see other people and diving weights and even the pattern on people’s bathing suits with crystal clarity.

    • Ellie

      Here’s one of your tedious “creationists” complementing you on a very fine and helpful article. Keep writing and here’s to us all seeing eye to eye one day.

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