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Scuba Diving and being able to see underwater [click here to return to previous page]

Water has a higher refractive index than air. Light entering the eye from the water behaves differently than light entering from air. This creates a distortion that affects normal vision by causing very severe hypermetropia. That is why people with severe myopia can see better underwater without a mask than normal-sighted people.

Diving masks and diving helmets and fullface masks solve this problem by creating an air space in front of the diver's eyes. The refraction error created by the water is corrected as the light travels from water to air, except that objects seem to be nearer than they are.  Divers who need corrective lenses to see clearly outside the water would normally need the same prescription while wearing a mask. Some masks can be ground to the diver's prescription to avoid the need for additional corrective lenses.  Occasionally commando frogmen use special contact lenses instead, to see underwater without the large glass surface of a diving mask which can reflect light and give away the frogman's position.  As a diver changes depth, he must periodically exhale through his nose to equalize the internal pressure of the mask with that of the surrounding water. Swimming goggles which only cover the eyes do not allow for equalization and thus are not suitable for diving. 

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Reference material for this scuba diving related informational article: wikipedia – the free online encyclopedia, scuba diving category

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