Which part of the eye controls the amount of light entering?

The iris is a thin, circular structure that surrounds the pupil in the eye and is primarily responsible for controlling the amount of light that enters the eye. It does this by adjusting the size of the pupil in response to changing light conditions. When in bright light, the iris constricts, making the pupil smaller to reduce the amount of light that can enter, thus protecting the internal structures of the eye and helping to prevent damage. In low light conditions, the iris dilates the pupil, making it larger to allow more light to enter, which enhances visibility in darker environments.

While the pupil acts as the opening through which light passes, it is the iris that actively regulates this opening's size. The cornea provides most of the eye's focusing power but does not control light entry directly. The lens further focuses light onto the retina but does not have a role in regulating light entry. Therefore, the iris is the critical structure for adjusting the light entering the eye, making it the correct answer in this context.