A Staff Report from the Straight Dope Science Advisory Board

Why is your vision blurred underwater?

September 5, 2002

Dear Straight Dope:

I'm sure people have noticed that, when swimming with your eyes open, your sight is quite a bit blurred, clear up close, but worsening rapidly with distance. But, when wearing diving goggles, you can see much better. Why? Does this apply to the eyes of fish as well? Is there a huge unexploited market for miniature diving goggles?

You never know. Maybe this is an opportunity for old dot-com entrepreneurs. But things don't look promising from a physiological standpoint.

The key here is the index of refraction. Index of refraction is a number which describes the speed of light through a transparent medium, and is defined as the ratio of the speed of light in a vacuum to the s.o.l. through the medium in question. For instance, light travels through water at about three quarters of its usual speed, so the index of refraction of water is about 1.33 . By contrast, light hardly slows down at all going through air, so the index of refraction of air is just slightly more than 1.

If light strikes the boundary of two materials (with different indices of refraction) at an angle, it'll pass into the other medium at a different angle. This is called refraction, and it's why a straw in a glass of water looks like it's bent at the surface of the water. If the boundary between the two surfaces is curved, then different light rays will be bent in different directions, and if the curvature is just right they'll be focused: You get a lens. Where exactly the rays focus depends on the amount of curvature and--here's the important part--on the ratio of the two indices of refraction.

In order for you to see things clearly, your eyes must focus light onto your retina. The human eye is wonderfully adapted for this purpose--but it depends on your looking through air. Under water, the shape and index of the lenses in your eyes stays the same, but the index of the stuff outside your eyes (the water) is now greater. As a result, light isn't refracted as much going into your eyes, and it focuses in a different place, so your vision is blurred. Fish, of course, have eyes adapted to this, so they can still focus underwater. A fish out of water, in addition to its other problems, would have blurred vision.

So how do goggles help? When you're wearing goggles, the medium in front of your eyes is air, just as your eyeballs expect. But there's also that boundary between the goggles and the water. Why doesn't that refract the light, too? Remember, lenses require curvature in addition to refraction, and the front of your swim goggles is flat. You'll still get refraction, but no focusing.

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