Pupil dilation is the process by which the pupil, the opening in the center of the iris, expands to allow more light to enter the eye. This reaction is controlled by the muscles in the iris and can occur in response to low light conditions, emotional responses, or certain medications. When the pupils dilate, it can enhance vision in dim environments. Conversely, they may constrict in bright light to protect the retina from excessive brightness. Pupil size can also be influenced by factors such as age, health, and the use of drugs.