AP Psychology Practice Test 2025 - Free AP Psychology Practice Questions and Study Guide

The function of the iris is indeed to control the size of the pupil opening. The iris is the colored part of the eye and plays a crucial role in regulating the amount of light that enters the eye by adjusting the size of the pupil. In bright light, the iris constricts the pupil to reduce light exposure, while in low light conditions, it dilates the pupil to allow more light in, enhancing visibility. This control of pupil size is essential for optimal vision in varying lighting conditions.

Other options refer to different functions that do not pertain to the iris. While shape-changing for image focusing is related to the lens of the eye, conversion of energy forms during sensation pertains to photoreceptor cells in the retina. Decreasing sensitivity to constant stimulation is related to sensory adaptation, not specific to the iris. Understanding these distinctions highlights why the correct answer focuses solely on the function of the iris in regulating light entry through the pupil.