If you've ever dunked your head underwater sans goggles in search of a set of keys or a pool toy, then you know how difficult it can be for the human eye to make sense of light in a subaqueous environment — but you might not know why this is.
The answer was recently explained with exceptional clarity by Michael Holcombe, in a piece that examines not only the optics behind humans' crummy underwater eyesight, but the evolution of subsurface eyesight in other species. Holcombe writes:
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.
Check out the rest of Holcombe's article for some thought provoking insights into the strange evolutionary history of eyesight — both below and above the surface of water.
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