Vision is affected greatly underwater. Objects will appear about a third larger and closer underwater. This is because of the way light refracts, or bends, through the mask lens. The magnification of an object depends on the following.
- Turbidity of the water
- Design of the mask lens
- Distance from the mask lens to the diver’s eyes
You need a mask because your eyes are designed to resolve an image in air. The ability of human eyes to focus depends on how the light enters the eye. When underwater, light enters a diver's eyes at a different angle, and if the scuba mask did not have an air gap, the diver would see objects that were out of focus. New divers may have a tough time adjusting to the magnification of near objects when underwater. After a few dives, they may hardly notice its effect.
Look at the image of the pencil in the glass of water. You can see how the pencil appears to be larger in the water.