The iris is a thin, circular structure in the eye that controls the size of the pupil and regulates the amount of light entering the retina. It is located between the cornea and the lens and is composed of two layers: the anterior layer, which contains muscle fibers, and the posterior layer, which has pigment cells. The color of the iris varies among individuals and is determined by the amount and type of pigment present. The primary function of the iris is to adjust the size of the pupil in response to light conditions. In bright light, the iris constricts the pupil to reduce light intake, while in dim light, it dilates the pupil to allow more light in. This process is essential for optimal vision and helps protect the retina from excessive light exposure.