What regulates the amount of light that enters the eye?

The iris is the part of the eye that regulates the amount of light entering the eye. It is a thin, circular structure that surrounds the pupil and contains muscles that contract and relax to change the size of the pupil. When the light is bright, the iris constricts the pupil to reduce the amount of light that enters, protecting the retina from excessive brightness. Conversely, in low light conditions, the iris dilates the pupil to allow more light to enter, enhancing vision in dim environments.

The other parts mentioned have distinct functions. The cornea serves as the primary source of light refraction and provides most of the eye's optical power. The lens further refines the focus of the light onto the retina but does not control how much light enters the eye. The pupil is the opening itself but is controlled by the iris; it does not actively regulate light intake by itself. Thus, the iris plays a crucial role in controlling light levels entering the eye.