The human eye is shaped like a ball with a bulge in the front. In the middle of this bulge is a hole called the pupil, which is the part we see as the black circle in the middle of the colored iris. The pupil lets in light to the eye. Behind it is the lens, which focuses the picture of what we can see. When light passes through the lens, it is turned upside down, so that an upside down picture is the image which hits the retina.
The retina is the part of the eye which actually makes sense of what we see. It is sensitive to light, and can sort out all the different colors we see. When light hits the cells of the retina, a chemical change takes place in the cells. This starts messages or impulses in the optic nerve at the back of the eye, which takes these messages to the brain. The brain turns the images the right way up, and identifies them as something we can recognize. Have you ever wondered why the pupils of the eyes grow bigger and smaller? This happens according to the amount of light available. In bright light, the pupil closes up to a black speck letting through only enough light to be able to see without damaging the retina. When the light is very dim, the pupil opens up to let in as much light as possible, so that, again, we can see.