What part of the eye is responsible for controlling the amount of light that enters?

The iris is the part of the eye responsible for controlling the amount of light that enters. It is a thin, circular structure located behind the cornea and in front of the lens. The iris contains two muscle groups, the sphincter pupillae and the dilator pupillae, which work together to constrict or dilate the pupil—the opening in the center of the iris. When the light is bright, the sphincter muscle contracts, reducing the size of the pupil to limit light entry, thereby protecting the retina from excessive brightness. Conversely, in low light conditions, the dilator muscle allows the pupil to enlarge, increasing the amount of light that can enter the eye to enhance visibility.

The cornea, while important for focusing light, does not actively control the light entering the eye but rather helps to refract light towards the retina. The pupil is the opening itself, rather than a structure that controls light. The eyelid serves primarily to protect the eye and help regulate light exposure, but it does not play a role in the physiological control of light entry in the same way the iris does.