dilation

The pupil is the opening in the center of the iris that allows light to enter the eye and reach the lens. Its size is controlled by the iris muscles, which constrict the pupil in bright light and dilate it in dim light. This adjustment regulates the amount of light entering the eye, optimizing vision and reducing aberrations.

Dilation, or the widening of the pupil, can occur in response to various stimuli. Strong emotional responses, such as fear, and physical stimuli like pain can trigger pupillary dilation. This is mediated by sympathetic nerve fibers. Conversely, focusing on near objects causes the pupil to constrict, while focusing on distant objects leads to dilation. The pupil's size can also be affected by age, disease, or trauma.12