opticsrefractionvisionwater
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 ?
the assertion that corrective lenses are not needed underwater is incorrect. diving masks with corrective lenses built-in have been in use for decades. the reason you yourself experience this effect is probably contained in Chris' and Martin's comments above.
Yes. I am myopic and I see a slight double image along the edge of my glasses:
This means that the field of view inside the frame is bigger - zoomed out - than what it would be with the same frames but without the lenses.
Similarly, objects are slightly but perceptibly smaller than they are without the glasses, as is clear from the size of the mugs at bottom left of the image. Zooming in on them and putting the inside- and outside-the-glasses versions in direct contrast gives a clearer picture:
As has been pointed out, the effect is strongly dependent on your exact position with respect to the lens, and it is exaggerated in the picture above because the camera is artificially far from the lens. Visually, and with the glasses correctly worn, the effect is milder and it looks closer to the door handle below:
For the handle to look like this I need to be about 2m from the door; I have about 2.5 dioptres in that direction if my memory serves.
In practice, though, your field of clear vision is restricted to the frame of the glasses, which do not cover a lot of your peripheral vision. This means that in practice you have a much reduced line of sight compared with someone without glasses, or who uses contact lenses. Whatever amplification was gained through this mechanism - the optics of which have been described in detail in other answers - is completely lost to this, and it's something to be aware of with people wearing glasses - our field of view is rather restricted, regardless of whether one is myopic or hyperopic.
Best Answer
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.
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
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.