Which structure in the eye controls the amount of light entering?

The iris is the structure in the eye responsible for controlling the amount of light that enters. It is a thin, circular structure located around the pupil and contains muscles that can contract or relax to adjust the size of the pupil. When the environment is bright, the iris constricts to make the pupil smaller, reducing the amount of light that enters the eye and protecting the retina. Conversely, in low light conditions, the iris dilates to enlarge the pupil, allowing more light in to enhance vision.

While the pupil itself is the opening through which light passes, it does not have the ability to change size on its own; this function is entirely dependent on the iris. The cornea primarily focuses light but does not control its quantity, and the lens adjusts focus but does not regulate light entry either. Understanding the role of the iris in this process is essential for comprehending how our eyes adapt to varying lighting conditions.