The eye is about the size of a golf ball. It has a lens inside which is as small as a pea. Light enters the eye through a hole called the pupil. From the outside this looks like a black dot in the middle of the eye. The light rays pass through the lens, which focuses them onto a thin layer, the retina, at the back of the eye. The retina contains light-sensitive chemicals which change the energy of light into the energy of tiny electrical nerve signals. These go along the optic nerve to the brain. About two-thirds of the information in the brain, about what we know and learn, comes in through our eyes as words and pictures. (World of Science)
