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Scuba Mask

Water has a higher refractive index than air, making someone with healthy vision unable to see well underwater without a scuba mask. Light entering the eye is barely refracted at all, only allowing a small part of the cornea called the crystalline lens to focus light. This causes a form of extreme farsightedness called hypermetropia, where the eye cannot focus on nearby objects.

Scuba masks solve this problem by having a pocket of air between the diver’s eyes and the water. People with myopia, or extreme shortsightedness, can actually see better underwater without a mask. The air corrects most of the problems, but objects still appear 34% bigger and 25% closer in salt water than they actually are. Adjustments must be made for hand-eye coordination and especially underwater photography, and field of view is severely limited.

A scuba mask must cover the diver’s nose to be effective. As a diver descends, more and more pressure is built up around the diver. Since humans are mostly water, the compression really only affects the areas that contain gasses, such as the lungs, eyes and sinuses. These areas are pressurized with the scuba tank and a scuba mask. As the diver descends, air is exhaled through the nose to pressurize the mask with an equal opposing pressure to the water. This is why goggles cannot be used for diving.

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