As the iris of the eye contracts and relaxes, an opening at its center, called the ________, grows larger or smaller, regulating the amount of light that enters the eye.

Iris

The colored part of the eye surrounding the pupil that controls the amount of light entering the eye.

Opening

An aperture or gap that allows access, passage, or a view through something.

Light

A type of electromagnetic radiation that is visible to the human eye, responsible for the sense of sight.