When you open your eyes under water, why is everything blurry?

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Are your eyes just focusing on “thin water”?

In: 5

So refraction is where light bends because it crosses a boundary between two materials that have different refractive indexes. The ratio between their indexes can predict how much the light _bends_ as it goes across that boundary.

The lens of and shape of our eyes are formed to focus the light very specifically on the inside of our retinaes. However their shape is predicated on the refractive indexes of the air/lens and lens/vitreous humour (the gel that fills your eye) and the various fluid/lens boundaries in between.

WHen you in water, that first air/lens boundary isn’t there, its a water/lens boundary and the refractive index of water is different than that of air… ; not by much but enough to make the light not focus where it should be.

Because light bends differently underwater than it does in air.
This is to say that its refractive index is greater than air.
Your eyes evolved with lenses to focus light that travels through air. And therefore are not adapted to focus light travelling through water.
They aren’t able to focus the light properly so objects look blurry.