The eye is very like a camera. It has a adjustable opening to let in light (the pupil), a lens which focuses the light to form an image, and sensitive film(the retina) on which the image is recorded.
Inside each human eye are about 130,000,000 light-sensitive cells. When light falls on one of these cellist causes a chemical change. This change starts an impulse in the eye fibre which sends a message through the optic nerve to the seeing part of your brain. The brain has learned what this message means so that we know exactly what we are seeing.
The eye itself is shaped like a ball with a slight bulge at the front. In the middle is a hole called the pupil, which appears black because it opens into the dark inside of the eye. Light passes through the pupil to the lens. The lens then focuses the light forming a picture at the back of the eyeball.