What is the role of the iris in the eye?

The iris plays a crucial role in regulating the amount of light that enters the eye by controlling the size of the pupil. This regulation is essential for optimizing vision under varying lighting conditions. In bright light, the iris constricts the pupil to decrease light entry, while in dim light, it dilates to allow more light in, facilitating better visibility.

The other functions described do not pertain to the iris. Focusing light onto the retina is primarily the responsibility of the lens, while protection from external damage is handled by the eyelids and other structures. Blood supply to the eye is provided by the blood vessels, not the iris itself. Therefore, the role of controlling the pupil size is distinct and vital, making it the correct choice in this context.