The structure and function of the eye are complex and fascinating. The eye constantly adjusts the amount of light it lets in, focuses on objects near and far, and produces continuous images that are instantly transmitted to the brain.
Light enters through the cornea, a transparent dome on the surface of the eye.
Besides serving as a protective covering for the front of the eye, the cornea also helps focus light on the retina at the back of the eye. The iris controls the amount of light that enters the eye by opening and closing like the aperture of a camera lens. The iris allows more light into the eye when the environment is dark and allows less light into the eye when the environment is bright. The size of the pupil is controlled by the pupillary sphincter muscle, which opens and closes the iris. Behind the iris sits the lens.