Underwater vision: how do we see beneath the waves?

Human eyes are not designed to focus well underwater, which is why objects appear blurry below the waves. On land, our eyes rely on refraction in order to focus correctly. You can observe refraction by placing a straw in a glass of water; from the surface it will appear to bend. This is because light travels in a straight line through a consistently dense medium, but when it passes between the air and a medium of different density, such as water, it bends or refracts at the boundary between the two, before continuing to travel on a straight path.

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The lens in a human eye works in a similar way, as refraction occurs when light travelling through the air passes into the cornea at the front, which has a much higher density. Unlike the flat surface of water, however, which bends all light rays in the same direction, the eye lens is curved, which means light is bent at different angles in order to create a focused image on the retina at the back. Human eyes can’t focus properly underwater because of this, as light rays travelling through the water are barely bent when they pass through the cornea and inside lens. This is why goggles give you a clearer view, as you’re placing a boundary of air between the water and your eyes, enabling refraction to occur.

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