The iris is the structure that controls the amount of light entering the eye. It is a thin, circular structure located right behind the cornea and in front of the lens. The iris contains muscles that can constrict or dilate the pupil, which is the opening in the center of the iris. When the iris muscles contract, they make the pupil smaller, reducing the amount of light that enters the eye, which is particularly helpful in bright conditions. Conversely, when the muscles relax, the pupil enlarges to allow more light in, which is advantageous in dim lighting.
Understanding the role of the iris involves knowing that it not only functions to regulate light but also contributes to the eye's overall appearance due to its pigmentation. The cornea, while crucial for focusing light as it enters the eye, does not control light admission; similarly, the pupil is an opening rather than a controlling structure. The lens focuses light onto the retina but does not regulate the amount of light that enters. Therefore, the iris is the correct choice in terms of controlling light entry.