Something about diving you need to know.

😀😀Nearly 3/4ths of the world is covered in water and what lies beneath is a sea life experience you can only enjoy if you’re looking. When you start this journey, you maybe need to know something about diving mask.

A diving mask (also half mask, dive mask or scuba mask) is an item of diving equipment that allows underwater divers, including, scuba divers, free-divers, and snorkelers to see clearly underwater. Surface supplied divers usually use a full face mask or diving helmet, but in some systems the half mask may be used. 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.

🌴🌴When the diver descends the ambient pressure rises and it becomes necessary to equalize the pressure inside the mask with the ambient pressure to avoid the barotrauma known as mask squeeze, This is done by blowing sufficient air out through the nose into the mask to relieve the pressure difference. This requires that the nose is included in the airspace of the diving mask. Equalisation during ascent is automatic as excess air easily leaks past the seal from inside.
Corrective lenses can be fitted to allow normal vision for people with focusing defects, or contact lenses may be worn inside the mask.
A wide range of viewport shapes and internal volumes are available, and each design will generally fit some shapes of face better than others. A good comfortable fit is important to the correct function of the mask.

Function:

🍡🍡Light rays bend when they travel from one medium to another; the amount of bending is determined by the refractive indices of the two media. If one medium has a particular curved shape, it functions as a lens. The cornea, humours, and crystalline lens of the eye together form a lens that focuses images on the retina. Our eyes are adapted for viewing in air. Water, however, has approximately the same refractive index as the cornea (both about 1.33), so immersion effectively eliminates the cornea's focusing properties. When our eyes are in water, instead of focusing images on the retina, they now focus them far behind the retina, resulting in an extremely blurred image from hypermetropia.By wearing a flat diving mask, humans can see clearly under water. The scuba mask's flat window separates the eyes from the surrounding water by a layer of air. Light rays entering from water into the flat parallel window change their direction minimally within the window material itself. But when these rays exit the window into the air space between the flat window and the eye, the refraction is quite noticeable. The view paths refract (bend) in a manner similar to viewing fish kept in an aquarium. Linear polarizing filters decrease visibility underwater by limiting ambient light and dimming artificial light sources.

Refraction of light entering the mask makes objects appear about 34% bigger and 25% nearer when underwater. Also pincushion distortion and lateral chromatic aberration are noticeable. As the diver descends in clean water, 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.

Construction:

👀👀Scuba diver with bifocal lenses in half mask
There are two basic categories of diving mask: The half mask covers the eyes and nose, and the full face mask covers eyes, nose and mouth, and therefore includes a part of the breathing apparatus. The half mask is described here.

Diving masks may have a single, durable, tempered glass faceplate, or two lenses in front of the eyes. These may be supported by a relatively rigid plastic or metal frame, or they may be permanently bonded to the rim of the skirt, in a construction known as "frameless". In the case of freediving masks, which need to have a low internal volume to minimize the amount of breath needed to equalize the change of pressure that occurs with depth, the lenses may be made of polycarbonate plastic. There is usually a "skirt" of synthetic rubber or silicone elastomer to support the frame and lenses and create a watertight seal with the diver's face. The skirt material may be almost transparent, translucent or opaque. A nearly transparent skirt provides a greater peripheral vision, though somewhat distorted, and may reduce the feeling of claustrophobia in some divers, but in some cases the light entering through the sides may cause distracting internal reflections. The skirt also encloses the nose, usually by means of a nose pocket, so that air can be exhaled through the nose into the mask to equalise the internal pressure during descent and thereby avoid possible barotrauma of the enclosed area of the face. The section of the mask covering the nose must allow the wearer to block the nostrils while equalising pressure in the middle ear. All diving masks have means to keep them in position, usually an elastomer strap of similar material to the skirt,[1] but occasionally an expanded neoprene pad with velcro straps is used. Mask straps are usually wider at the back or split into an upper and lower strap at the back of the head for stability and comfort.
Some masks had a one-way purge valve under the nose to let water out, but this is no longer common as they were neither necessary nor reliable, and often leaked
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Visual correction

👧Diving masks can be fitted with prescription lenses for divers needing optical correction to improve vision. Corrective lenses are ground flat on one side and optically cemented to the inside face of the mask lens. This provides the same amount of correction above and below the surface of the water. Bifocal lenses are also available for this application. Some masks are made with removable lenses, and a range of standard corrective lenses are available which can be fitted. Plastic self-adhesive lenses that can be applied to the inside of the mask may fall off if the mask is flooded for a significant period. Contact lenses may be worn under a mask or helmet, but there is some risk of losing them if the mask is dislodged in turbulent water.

Fit:

😊😊A mask is considered to fit well when it seals comfortably and effectively all around the skirt, and provides a good field of vision, without pressing on the nose or forehead while in use underwater. A low internal volume is considered desirable by freedivers, as less breath is wasted to equalise, and by scuba divers, as there is less tendency to press up under the nose due to buoyancy, which becomes uncomfortable quite quickly.

Divers may test whether a mask is a good fit by placing it on their face, without using the straps, and gently inhaling through their nose. If the mask stays on without any help this indicates that no air is being drawn in and that the skirt is in sufficient contact with the facial skin all the way round the mask. Optimum sealing requires that hair strands do not cross under the edge of the seal, as they can provide a path for water to leak into the mask. This is more a problem with the forehead hairline than with lower facial hair, as water from the top tends to run in

to the eyes, while water pooling at the bottom is easily purged by exhaling a small volume of air through the nose. Most bearded divers learn to manage the slight leakage without difficulty, inconvenience, or greasing the moustache. Other factors affecting a comfortable fit are sufficient space for the nose in the nose pocket, no contact between the rigid mask frame and the bridge of the nose, and no excessive pressure on the forehead. There should also be sufficient space between the mask lenses and the face that the eyelashes do not contact the glass noticeably when blinking.

👄👄😀😀Next time, we'll share something about type of diving mask, using and make clearing.