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Yesterday I looked underwater with my eyes open (and no goggles) and I realized I can't see anything clearly. Everything looks very, very blurry. My guess is that the eye needs direct contact with air in order to work properly. With water, the refraction index is different, and the eye lens are not able to compensate for correct focalization on the retina.

Am I right ? If so, what lenses should one wear in order to see clearly while under water ?

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4  
Some animals can see both in water as well as in the air, with no fuzzyness. I wonder how??? –  ja72 Jan 28 '11 at 2:49
    
I would guess that water has more local density variations than air and this is also a big part of the problem, can anyone comment on that? –  ptomato Jan 28 '11 at 8:49
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@ptomato I don't think the density of water changes much at all, especially compared to a gas. Wikipedia suggests that water compresses about as readily as solid rock in the Compressibility article –  Brian Gordon Sep 28 '11 at 2:17

4 Answers 4

up vote 19 down vote accepted

You can't see clearly underwater for a couple of reasons. One is the thickness of your lens, but the main one is the index of refraction of your cornea.

For reference, here's the Wikipedia picture of a human eye.

http://en.wikipedia.org/wiki/File:Schematic_diagram_of_the_human_eye_en.svg

According to Wikipedia, two-thirds of the refractive power of your eye is in your cornea, and the cornea's refractive index is about 1.376. The refractive index of water (according to Google) is 1.33. In water, your cornea bends light as much as a lens in air whose refractive index is

$$\frac{1.376-1.33}{1.33} + 1 = 1.034$$

That means you're losing about 90% of your cornea's refractive power, or 60% of your total refractive power, when you enter the water.

The question becomes whether your lens can compensate for that.

I didn't find a direct quote on how much you can change the focal distance of your lens, but we can estimate that your cornea is doing essentially nothing, and ask whether your lens ought to be able to do all the focusing itself.

For a spherical lens with index of refraction $n$ sitting in a medium with index of refraction $n_0$, the effective focal length is

$$f = \frac{nD}{4(n-n_0)}$$

The refractive index of your vitreous humor is about 1.33 (like water), and the refractive index of your lens, according to Wikipedia, varies between 1.386 and 1.406. Let's take 1.40 as an average. Then, plugging in the numbers, the effective focal distance of a spherical eye lens would be five times its diameter.

The Wikipedia picture of a human eye makes this look reasonable - a spherical lens might be able to do all the focusing a human eye needs, even without the cornea.

The problem is that your eye's lens isn't spherical. From the same Wikipedia article

In many aquatic vertebrates, the lens is considerably thicker, almost spherical, to increase the refraction of light. This difference compensates for the smaller angle of refraction between the eye's cornea and the watery medium, as they have similar refractive indices. [2] Even among terrestrial animals, however, the lens of primates such as humans is unusually flat.[3]

So, the reason you can't see well underwater is that your eye lens is too flat.

If you wear goggles, the light is refracted much more as it enters the cornea - the same amount as normal. If you want to wear some sort of corrective lenses directly on your eye like contact lenses, they should have a refractive index as low as possible.

Googling for "underwater contact lens", I found an article about contact lenses made with a layer of air, allowing divers to see sharply underwater.

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Which is essentially saying the the original questioner is correct. It is the ratio of refractive indices that matter. Goggles provide an air gap that allows an "air" lense to work properly. –  Omega Centauri Jan 27 '11 at 22:09
    
Beautiful answer @Mark, +1. –  user346 Jan 28 '11 at 19:23
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According to the BBC documentary "Inside the human body" people can control their pupils to get as small as possible while underwater. Increasing depth of field, hence sharpening the out of focus image. Supposedly kids can learn this easily. –  Kris Van Bael Jan 3 '12 at 7:07

Am I right ?

Yes.

If so, what lenses should one wear in order to see clearly while under water ?

You don't need extra lens you have one in your eyes, just use goggles that makes a layer of air between the water and your eyes.

If you decide to put a convergent lens in front of your eyes it won't work because your eye will still not be able to focus different depths. So, you will only be able to focus rays coming from a fixed distance, what is not very useful.

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Snell’s law works with a change in the index of refraction across two media. The difference is greater for air on cornea than it is for water on cornea.

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The reason goggles work is because the lenses are flat. That way the interface between air and water doesn't have any focal length properties.

Another way of saying the same thing: You can see fish perfectly clearly in an aquarium with flat sides. The only effect is that the fish seem a little closer than they actually are. Goggles do the same thing for the same reason.

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protected by Qmechanic Jul 7 '13 at 6:48

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