Which structure regulates the amount of light entering the eye?

The pupil is the structure that regulates the amount of light entering the eye. It is essentially an opening in the center of the iris, which is the colored part of the eye. The size of the pupil can change in response to various lighting conditions; it constricts in bright light to reduce the amount of light that enters, protecting the retina from excessive brightness and potential damage. Conversely, in dim light, the pupil dilates to allow more light into the eye, enhancing visibility in darker environments.

The lens is involved in focusing light onto the retina, but it does not regulate light entry. The ciliary body controls the shape of the lens for focusing but does not control the amount of light entering. The sclera provides structural support for the eye and is not involved in the regulation of light. Thus, the function of controlling light entry lies specifically with the pupil.