Here’s one I haven’t figured out…
Why do objects appear blurry underwater? When I dive into one end of my backyard swimming pool, I can’t read a hand-lettered sign (letters are 6" high, distance ~40’) on the other end of the pool, until you give me a diving mask, in which case everything is perfectly clear and readable.
I’m having trouble understanding why my vision is clearer with the diving mask. Without the mask, my eyes make direct contact with the water (the medium through which the light travels), so shouldn’t sight be as clear as it is when using a mask?
I’m pretty sure this one’s come up on the SDMB before, but it’s a fun question, so here goes.
Although you have a Lens in your eye, most of the optical power actually comes from the bending at the cornea/air interface. It’s not surprising that this is so – the optical power (inverse of the focal length) varies as the difference between the indices of refraction on both sides of the interface. The cornea and the lens and the vitreous humor in your eye have indices of refraction that are pretty close to that of water, so the biggest amount of ray bending happens where the indices are the most different. Air has an index of 1.00 . Your cornea has an index pretty close to 1.3 .
It isn’t just your eye that this happens to – drop a glass lens in water and its focal length gets longer, too, and its power gets smaller.
You can see if you’re wearing a mask, because you still have that air-cornea interface if your mask is full of air. You then look through the glass window in the front as if you’re looking in through the wall of an aquarium.
There’s no reason that you couldn’t make special glasses or contacts that would let you see in the water, but there’s not much call for it. Most people don’t like the feel of water on their eyes, and would rather wear air-filled goggles.
When you open your eyes underwater everything looks blurry because you’ve suddenly made the effective focal length of your eyes longer, so the image doesn’t form on your retina – it’s out of focus. If your retina weren’t in the way, the image would come to focus behind your eye.
Diving and the Eye
Basically, it’s due to refraction. Light travels more slowly through water than through air. The effect is that the light bends when it moves from the water to the air between your eye and the mask lens. With air on both sides there is no magnification; the light rays don’t bend.
With water on one side of the mask, the light’s path will bend (refract) the more off-center you look. That’s why objects look bigger and closer underwater (about 30% magnification). Underwater without a mask, no refraction occurs, so your vision will remain the same as on land.
I have extreme myopia (20-400 in one eye, 20-700 in the other) and as a lad I found that my vision improved underwater. So much so that there were summers when I’d spend as much time underwater as I could, sometimes resorting to a snorkel.
As an adult, I’ve been known to remove my mask while scuba diving, so I could see better.
Fortunately, for those times when I can’t conduct affairs while submerged, contact lenses correct my vision rather well.