What part of the eye adjusts the size of the pupil?

The iris is the part of the eye responsible for adjusting the size of the pupil. It is a thin, circular structure that surrounds the pupil and contains muscles that contract or relax in response to changes in light intensity. When light levels are high, the iris constricts the pupil to limit the amount of light entering the eye, protecting the internal structures and improving visual acuity. Conversely, in low light conditions, the iris dilates the pupil to allow more light to enter, helping to enhance vision in darker environments. This dynamic adjustment mechanism is crucial for optimal vision under varying lighting conditions.

The sclera, being the white outer layer of the eye, provides protection and structural support, but does not influence pupil size. The cornea, which is the transparent front part of the eye, primarily focuses light onto the lens but does not adjust the pupil. The lens, located behind the iris, changes shape to focus light on the retina but is not responsible for pupil size adjustment. Therefore, the iris is the correct choice when discussing the adjustment of the pupil size.