The diving mask is diver in the water window.
Due to the density of water is bigger than air, human eyes in the water cannot be directly focus, so divers must wear mask, the front of the eyes keep the air cavity, so divers in the water can be normal viewing.
Try to open your eyes in the water, can see only a blurred sight, this is because the density of water is bigger than air, the light into the water will have refraction. While the focal length of the eyes is based on the air light to adjust, therefore, diving mask make your eyes kept cavity, let you have a clear line of sight. Diving mask and swimming glasses the biggest different depend on the former not only cover the eyes still cover the nose, this is because when dives in order to prevent extrusion, need to balance the pressure inside the nasal cavity, so swimming glasses can't used for diving.
The general diving mask is composed by strengthening safety glass mirror, joint face rubber or silica gel group of edge and fixed position adjustment headband. Reinforced safety glass lenses can prevent broken into a highly dangerous slender glass fragments. Group of edge the material of silica gel is stronger than the rubber, which is because the silica gels than rubber durable 3 to 4 times, is not easy to skin allergies, more soft and comfortable. Some mask no drain valve, is a one-way valve used to eliminate the stagnant water inside the mask. The two most important points of selection diving mask is whether suitable and comfortable or not, other equipment is also true. An inappropriate mask might leak may also cause allergies and reduce a lot of the fun of diving.
Test diving mask whether appropriate or not, as long as gently put the mask on the face, (do not have to wear the headband) then inhale with the nose, suitable mask will be close to the face until you exhale. Another point to note is that, to make sure you can easily in mask outside hold your nose. Mask style is varied, have monolithic lens, two lens and multichip lens are many, usually only two pieces of lens design with an adjustable corrective lenses.