The pupil is the opening in the center of the iris, the colored part of the eye. It allows light to enter the eye and reach the retina, where images are formed. The size of the pupil can change in response to light conditions; it constricts in bright light and dilates in dim light to help regulate the amount of light that enters. Pupil size is controlled by two sets of muscles in the iris: the sphincter pupillae, which constricts the pupil, and the dilator pupillae, which enlarges it. This adjustment helps improve vision and protects the retina from excessive light exposure.