The part of the human eye that controls the amount of light entering is called the iris. The iris is a thin, circular structure that surrounds the pupil, the opening at the center of the eye. By contracting or dilating, the iris regulates the size of the pupil, thus controlling how much light reaches the retina at the back of the eye. This adjustment is crucial for optimal vision in varying lighting conditions, enabling the eye to adapt to bright daylight as well as dim environments. The iris also gives the eye its color, which can range from blue to brown or green, depending on genetic factors.