ABSTRACT
The eyes continually adjust the amount of light they let in, change their focus on objects far and near, and produce continuous images, which they instantly transmit to the brain. After passing through the cornea, light enters the pupil, the opening that appears as a small round black area in the middle of the iris. The pupillary dilator and sphincter muscles open and close the pupil to control the amount of light that enters the eye: letting more light in when the environment is dim but admitting less light when the surroundings are bright. The eyes of a healthy young adult can usually focus on an object as close as 10 cm, but with aging, the lens becomes less flexible and is hence less able to thicken, which diminishes its ability to deal with nearby objects. Even for fully functioning eyes, the perception of colour requires sufficient light energy arriving at the cones to activate them.
