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.
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.
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?
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.
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.
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.