Why is your vision so fuzzy when you open your eyes underwater? The surface of your eye must remain wet all of the time, but this natural moisture doesn’t make your vision 20/100, so why does water? Does salt vs. fresh water make a difference in visual clarity underwater?
Well, light refracts where two materials of different densities meet. The greater the difference in density (or more specifically, the index of refraction) the more the light bends. The air/eye interface is a big jump in density, so light bends effectively. But once you are in water, you don’t have an air/eye interface, you have a water/eye interface. The density jump is much smaller, so light hardly refracts at all. So your eyes are unable to focus the light onto the retina.
Try dipping a magnifying glass in a tub of water and see if it still works. It won’t, for the same reason.
Thanks, scr4.
Actually I can see better underwater since I am myopic. Since the index of refraction in water is different that in air, the focus point is closer to my retina underwater than it is in air.
So I wonder if a sea creature like a whale or fish would see a big blur when their eyes are out of the water? There are fish that have an iris shaped like a figure 8; they use the lower part to see underwater, and the upper to look out through the air - I wonder how the lense/s work in such an eye? I am a fish biologist, so I should know… but they didn’t cover that in class.
To add to scr4’s post…
If you wear swimmer’s goggles underwater, you can see clearly thanks to the restored air/eye interface. Then it’s just a matter of water clarity.
I would imagine that whales and other sea creatures have corneas that are shaped to give perfect imaging even in water (the cornea doesn’t have the same index as water, so there’s still some optical power). As for those fishes that have eyes half above-half below water, I’d be willing to bet that their lenses are shaped differently below the water than above the water. The iris has much less effect than you’d think – those eyes would probably work almost as well with circular irises as with figure-8 irises.