Which part of the eye controls the size of the pupil opening?

The iris is the part of the eye that regulates the size of the pupil opening, which is essential for controlling the amount of light that enters the eye. The iris is made up of muscular fibers that can contract and expand, allowing it to adjust the size of the pupil in response to varying light conditions. When light is bright, the iris constricts the pupil to limit the amount of light entering the eye and protect the sensitive retinas. Conversely, in low light conditions, the iris dilates the pupil to allow more light in, enhancing visibility.

The pupil itself is merely the opening through which light passes; it does not have the ability to change size on its own. The cornea is primarily responsible for focusing light onto the retina and does not play a role in adjusting the pupil size. The lens also contributes to focusing light, but like the cornea, it does not influence pupil size. Thus, the correct identification of the iris emphasizes its crucial role in the dynamic adjustment of the pupil.