People can not see underwater clearly. A person who opens his eyes in a swimming pool or in the ocean will be treated to a distorted, blurry image. Scuba divers, who must be able to see underwater to read their gauges and communicate with hand signals, use scuba masks. How does a scuba mask help people to see underwater?
The answer lies in the fact that light bends (or refracts) when it travels between two different substances. For example, light will bend and change its direction slightly when it travels between air and water or between air and a person’s eye.
What is interesting is that a light wave will bend at a different angle depending upon what two substances it travels between. For example, light traveling between air and an eye bends at one angle, while light traveling between water and an eye bends at a different, smaller angle.
The construction of a human eye allows it to clearly focus only on a specific angle of light: the angle that light bends when it travels between air and the eye. When light travels between water and a person’s eye, the eye can not focus correctly because the light is bent at the wrong angle. This is the reason that people have blurry, unfocused vision when they open their eyes underwater.
A scuba mask helps a diver to see underwater because it holds air against his face. Light waves travel from the water, through the mask’s glass, through the air inside the mask, and then into his eye. When light travels to a diver’s eyes through the air inside the mask, it bends a the proper angle and his eyes can focus.
Remember, light waves bend when they travel between air and water. The light beam that finally reaches a diver’s eye has traveled through water, glass, and air before entering his eye. This process bends the light, but only a little. Because the light enters the air in a diver’s mask at a slight angle, vision through a scuba mask is a bit distorted. Objects seen through a scuba mask appear about 33% bigger than they do in air.
Natalie Gibb of About.com