Eye Masks

A diving mask is an item of diving equipment that allows scuba divers, free-divers, and snorkelers to see clearly underwater. more...

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When the human eye is in direct contact with water as opposed to air, its normal environment, light entering the eye is refracted by a different angle and the eye is unable to focus the light. By providing an air space in front of the eyes, light enters normally and the eye is able to focus correctly.

Construction

Diving masks have a durable, tempered glass plate in front of the eyes and a "skirt" of rubber or silicone to create a watertight seal with the diver's face. A strap keeps the mask in position.

Masks that are used at depth must be constructed so that the diver can exhale through the nose into the mask to prevent the "squeeze" caused by increasing pressure during descent in water.

Some masks have a one-way outlet valve under the nose to let water out. The diver simply holds the mask upright and exhales through the nose. Other masks have ways to hold the nose closed to assist in equalizing the pressure inside the ears.

Sometimes masks are sold in conjunction with snorkels and/or swimfins. Low quality snorkelling masks may have a plastic or low-quality glass faceplate, and are not recommended for anything but rare, casual use. Masks that have the snorkel built in are considered hazardous as well.

The best way to find out if a mask is a good fit is; (while facing forward), place it on your face, (without the strap), and, (without using your hands to hold it), inhale gently thru your nose. If the mask stays on, (and air doesn't get in), then the fit is good. If the mask falls off, then try another. Tightening the strap will not seal a mask that doesn't fit.

Use

To prevent a mask from fogging up due to condensation on the glass plate many divers spit into the mask, wipe the spit around the inside of the plate and wash it out with a little water. There are commercial products that can be used as an alternative to the saliva method.

Refraction of light entering the mask makes objects appear bigger and nearer underwater. As the diver descends, the water acts as a colour filter eliminating the red end of the visible spectrum of the sunlight entering the water leaving only the blue end of the spectrum. Depending on the depth and clarity of the water, eventually all sunlight is blocked and the diver has to rely on artificial light sources to see underwater.

A variety of prescription lenses can be fitted inside the glass plate of the mask to correct some visual problems underwater. Divers may able to use contact lenses inside the mask but they must keep their eyes closed if they remove the mask underwater to avoid losing the lenses.

Read more at Wikipedia.org