What part of the eye controls the amount of light entering?

The iris is the part of the eye that regulates the amount of light that enters by adjusting the size of the pupil, which is the opening in the center of the iris. The iris contains muscles that can constrict or dilate the pupil in response to different lighting conditions. In bright light, the iris contracts, making the pupil smaller to limit the amount of light that can enter the eye and protect the retina. Conversely, in low light, the iris relaxes, allowing the pupil to widen and let more light in, enhancing visibility. This function is crucial for maintaining optimal vision under varying lighting conditions.

The ciliary body is involved in the process of accommodation, helping to focus light on the retina, while the cornea primarily refracts light as it enters the eye. The pupil is simply the aperture through which light passes, but it is the iris that actively controls its size.