Optical refraction describes the effects of light moving more slowly through one substance than another. Water has substantially greater refractive properties than air.
The Human Eye
The human eye has evolved to focus light under the refractive conditions present in ordinary air. The greater refractive properties of water interfere with the workings of the eye, which is why everything underwater seems blurry and out of focus.
The principal function of a swimming mask is to provide a pocket of air, allowing the eyes to focus normally. This makes them useful to ordinary swimmers, but critical to snorkelers and divers.
As water depth increases, so does water pressure, which pushes onto the swimming mask. Divers, as well as snorkelers making skin dives, must add extra air to the insides of their masks by exhaling a little through the nose to equalize the pressure and prevent getting "mask face" or even breaking the mask.
It is the nose piece of the swimming mask that makes it more suited to doing deep relative to mere goggles, as the nose pieces allows a swimmer to equalize the pressure inside the mask. This is impossible with goggles.
Swimming goggles are sometimes also called swimming masks, and while this is common it is also somewhat misleading. Goggles, as previously noted, lack the nose piece of the swimming mask.
Article Written By Edwin Thomas
Edwin Thomas has been writing since 1997. His work has appeared in various online publications, including The Black Table, Proboxing-Fans and others. A travel blogger, editor and writer, Thomas has traveled from Argentina to Vietnam in pursuit of stories. He holds a Master of Arts in international affairs from American University.