What is the primary role of the iris in the eye?

The primary role of the iris in the eye is to control the size of the pupil. The iris is a thin, circular structure that surrounds the pupil and is made up of smooth muscle fibers. These fibers contract and relax to adjust the diameter of the pupil in response to varying light conditions. This action helps regulate the amount of light entering the eye, which is crucial for optimal vision. When it is bright, the iris constricts the pupil to limit the light; in dim conditions, it dilates the pupil to allow more light to enter. This function is vital for protecting the retina from excessive light and enhancing visual acuity across different lighting environments.

While other options mention important functions related to the eye, they do not reflect the specific role of the iris. For instance, producing tears is the function of the lacrimal glands, converting light into neural signals is handled by the retina, and protecting intraocular structures involves different parts of the eye and other mechanisms.