The pupil's a circular opening located in the center of the iris of the eye that controls the amount of light that enters the eye. It appears black because most of the light entering the pupil's absorbed by the tissues inside the eye. In optical terms, the anatomical pupil's the eye's aperture and the iris's the aperture stop. The image of the pupil as seen from outside the eye's the entrance pupil, which doesn't exactly correspond to the location and size of the physical pupil because it's mag… (More on Pupil)