When you shine a light into your friend's eye in a dimly lit room, the pupil undergoes a reflexive response known as the pupillary light reflex. The iris, the colored part of the eye, contains two sets of muscles—radial muscles and circular muscles. When light is introduced, the circular muscles contract, causing the pupil to dilate and allowing more light to enter the eye. This is a protective mechanism to optimize vision in low-light conditions.