The pupil is the opening in the center of the iris that allows light to enter the eye. Its size can change in response to various factors, including light levels and emotional states. In bright light, the pupil constricts to limit the amount of light entering, while in dim light, it dilates to allow more light in for better vision. Pupil size can also be influenced by certain medications, drugs, and health conditions. For example, opioids can cause constricted pupils, while stimulants may lead to dilated pupils. Monitoring pupil size can provide insights into a person's neurological function and overall health.