What part of the eye controls the amount of light that enters?

The iris is the part of the eye that controls the amount of light entering through the pupil. It is a thin, circular structure that surrounds the pupil and contains muscles that regulate its size. When light levels are bright, the iris constricts the pupil to limit the amount of light that can enter, protecting the inner structures of the eye and helping to improve vision clarity. Conversely, in dim lighting, the iris relaxes, allowing the pupil to dilate and let in more light, thus enhancing visibility in lower light conditions.

Other structures like the cornea and lens play important roles in focusing light and refracting it to form clear images on the retina, but they do not directly control the quantity of light entering the eye. The cornea primarily provides the majority of the eye's focusing power, while the lens adjusts the focus for objects at different distances. The pupil itself is merely the opening that allows light to pass through, rather than a structure that regulates light entry. Thus, the iris is crucial for managing how much light reaches the retina, making it the correct answer.