I’ve dealt with this problem all my life – i have high powered glasses (-6 and – 8, with some cylindrical /spherical tweaks). My glasses work great. But i wore them while swimming in the crystal blue Mediterranean, and when I’m diving in, i realized the glasses aren’t effective. I understand the wavy reflective patterns interrupting, but why cant I focus on anything easily?
In: Physics
Water is not air
No mater how “crystal blue” the water appears; it’s still not air.
Water is denser; reflects/refracts light differently
You can see for miles looking out across the horizon; in even the clearest water only a few dozen meters
Water doesn’t really work with glasses.
Glasses work by refracting* light at a specific angle to essentially offset your vision impairment, which is a sort of distance-distortion due to shape your eyes + lenses. But, light travels differently through water than through air, and the interface between air/water and glass essentially defines >how< that light is bent.
So in water, it will *not* bend the way it’s supposed to in air. Hence, it no longer properly corrects your vision.
To do their thing your glasses have to (de)focus light in a specific way to correct for your eyes. The change in direction happens when light changes the material in travels in: normally from air to glass/plastic, then back to air, and into your eyeball.
But the change is stronger the more different the materials are*. Air is quite different from the glasses, so it changes significantly. But if there is only water, no air, then the change is much less. So the light is changed only a bit and the glasses cannot properly do their job anymore.
It also doesn’t help that the change from air to eyeball (mostly water) is now essentially gone, too.
*: more precisely it depends by what factor the speed of light changes, denser stuff typically having slower speeds.
You can get prescription goggles, which do work underwater, possibly because there’s not water between the lens and your eye
The way your glasses work is (very basically) by bending light towards more the front or back of your pupils. Glasses can do this because the refractive index of glass is different from the refractive index of air. The refractive index is how we describe how much light bends as it passes through a transition to different mediums: the bigger the difference, the more it bends. Combined with the shape of the lenses, we can bend the light more towards the point in your eyeball where it is most clearly seen.
Water also has a different refractive index to air, which is why things in a swimming pool can often appear magnified. Water’s refractive index is closer to glass than air’s is, so the difference between their refractive index is smaller, and thus light bends less as it moves from one to the other. This changes where the point the light it bent towards is, thus making your glasses less effective.
If you wear them inside goggles, they do work, but that may also let water leak into the goggles.
Every curved boundary between two materials matters here. The boundary between your eye and the air will bend light less underwater, and the boundary between the lens and the air will bend light less underwater, because the difference in refractive index is smaller between water and eye/lens than between air and eye/lens.
Glasses work due to their chape. If the glasses were surrounded by the same glass, they would not have the necessary shape (because you couldn’t distinguish glasses-glas from surrounding glass). For water, it is the same, just to a lesser degree.
ELI5 answer: water is like another pair of glasses. Except it’s a very messy, random prescription.
If you put multiple pairs of glasses on, it doesn’t work very well.
The focusing power of a lens depends on a few things. The diameter and the curvature of your glasses don’t change when you put them in water. Neither does the refractive index of the lens. What does change is the refractive index of the surrounding material.
Light is bent when it goes from one material to another. How much the light bends depends on the difference of their refractive indices. Refractive index is how quickly light travels in a material.
Water has a higher refractive index than air, so the light doesn’t bend as much when it goes from water to glass as it does when it goes from air to glass. That’s why your lenses don’t have as much power underwater. The power of a lens depends on the material surrounding it.
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