Why when we look at water it’s clear. but when we go under water it’s blurry?
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The proper function of our eyes depends on there being air on the outside of our eye’s lens. The shape of our eye, the lens, cornea etc. are all designed to focus light very precisely on the retina at the back of our eyes.
When you are near or far sighted and need glasses or some other type of correction, its because, for one or more of some very good reasons (age, growth, atrophy and their derivative conditions: glaucoma, lens stiffness, eye shape, whatever etc.) the light isn’t being focused in the right spot.
Refraction is where light _bends_ as it transitions between two different but still transparent (or light carrying) materials. You can see this when you put a straw into a glass of water – above the water its at a certain angle, below, its at a different angle (or so it would seem, as you look at it from above the glass).
The shape of our eyes, the lens in particular, has evolved to focus the incoming light on our retinas accounting for the refraction between air and the material of our eye’s lenses (and then the clear goo that fills our eyeballs). If you replace the air with water, that amount of refraction goes bork and the light you see through your eyeballs is no longer focusing quite in the right spot.
Having said that, I bet some – very few – people who DO need corrective eye-wear out of water, might actually be able to see _better_ underwater. But I’m not an optometrist so I’m speculating.
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