The part of the human eye that controls the amount of light entering is the iris. The iris is the colored portion of the eye surrounding the pupil, which is the opening that lets light in. The iris contains muscles that contract and dilate to adjust the size of the pupil in response to lighting conditions. In bright light, the iris constricts to reduce the amount of light that enters, protecting the retina from damage. Conversely, in low light, the iris dilates to allow more light in, enhancing visibility. This regulation of light is crucial for clear vision in varying environments.