Underwater vision. Water has a higher refractive index than air - similar to that of the cornea of the eye. Light entering the cornea from water is hardly refracted at all, leaving only the eye's crystalline lens to focus light. This leads to very severe hypermetropia. People with severe myopia, therefore, can see better underwater without a mask than normal-sighted people. Diving masks and helmets solve this problem by providing an air space in front of the diver's eyes. The refraction error created by the water is mostly corrected as the light travels from water to air through a flat lens, except that objects appear approximately 34% bigger and 25% closer in salt water than they actually are. Therefore total field-of-view is significantly reduced and eye hand coordination must be adjusted. (This also affects underwater photography: a camera seeing through a flat port in its housing is affected in the same way as its user's eye seeing through a flat mask viewport, and so its operator must focus for the apparent distance to target, not for the real distance). Divers who need corrective lenses to see clearly outside the water would normally need the same prescription while wearing a mask. Generic and custom corrective lenses are available for some two-window masks. Custom lenses can be bonded onto masks that have a single front window or two windows. A "double-dome-ported mask" has curved viewports in an attempt to cure these faults, but this causes a refraction problem of its own. . Commando frogmen concerned about revealing their position when light reflects from the glass surface of their diving masks may instead use special contact lenses to see underwater. As a diver descends, he must periodically exhale through his nose to equalize the internal pressure of the mask with that of the surrounding water. Swimming goggles are not suitable for diving because they only cover the eyes and thus do not allow for equalization. Failure to equalise the pressure inside the mask may lead to a form of barotrauma known as mask squeeze. (1) Light underwater: Water preferentially absorbs red light, and to a lesser extent, yellow and green light, so the color that is least absorbed by water is blue light. (2) Table of Light Absorption in pure water: Color Average wavelength Approximate depth of total absorption: Ultraviolet 300 nm 25 m, Violet 400 nm 100 m, Blue 475 nm 275 m, Green 525 nm 110 m, Yellow 575 nm 50 m, Orange 600 nm 20 m, Red 685 nm 5 m, Infra-red 800 nm 3 m