How is the size of the pupil regulated

The pupil is located in the center of the iris. To be more exact, it is a hole located in the center of the iris. Its primary job is to regulate how much (or how little) light enters the eyes. This is done through muscles in the iris, which will constrict or enlarge the pupil. Nerve impulses travel down the optic nerve after light enters the eye, which affects the size of the pupil. The pupil changes involuntarily. It is known as the pupil reflex.

The amount of light reuglates the size of ones pupil