The human eye sees very well in air. In water, though, it doesn't do so well. This is due to refraction. Light bends as it passes from a material with one density to a material with a different density. Incoming light rays have to land precisely on the back of the eye. The eye's lens bends some of the light. However, about 70 percent of the bending happens before the light reaches the lens, when the rays pass from the air outside the eye to the more dense, fluid-filled eyeball. Vision is difficult underwater because water is almost the same density as the fluid inside the eye. Light barely bends as it enters the eye, and the result is blurred vision. The Moken are a tribe of skilled divers who live off the west coast of Thailand. They are also known as "sea gypsies." They harvest clams, sea cucumbers, and other marine animals from deep under the water, sometimes as much as 75 feet beneath the surface! But without goggles or other aids, the gypsies can spot even the smallest of shellfish -- ones that most people would be unable to distinguish from surrounding pebbles. The Moken are not born with this talent. They learn to do it by shrinking the size of their pupils when they are very young. Other animals, such as fish and frogs, can also see underwater, but they use a different method. They flatten the covering in front of the eye to help bend the light more.
