When you open your eyes underwater, things look blurry, but with goggles, everything looks perfectly clear. Why can't we landlubbers focus underwater without goggles?
First, we need to explore how our eyes focus when we're high and dry. Our ability to focus depends on the refraction, or bending, of light rays as they pass between substances of different densities. One way to see refraction is to put a spoon in a glass of water. It looks like the spoon bends at the water's surface, but it's really the light rays bending as they pass between the air and the denser water. A lens works in a similar way. However, unlike the flat surface of water, which bends all the rays the same way, a lens is curved, bending rays at different angles so they come to a focus.
The front of the eye has two lenses, the cornea on the outside and another lens inside, to focus images on the retina, at the back of the eye. Most of the refraction takes place when light passes from the air into the cornea, which is much more dense, but water and the cornea have similar densities. So, when we open our eyes underwater, incoming light rays are hardly bent, or focused, at all. The inside lens bends the rays a little, but it can't make up for the lost corneal refraction, so the light that reaches the retina isn't focused and the underwater world looks blurry.
Goggles clear things up by inserting a pocket of air in front of the cornea, restoring that crucial interface between substances of different densities to refract the light.
