What part of the eye controls the amount of light that enters?

The part of the eye that controls the amount of light entering is the iris. The iris, which is the colored part of the eye, contains muscles that adjust the size of the pupil. When the iris contracts (in bright light), it reduces the size of the pupil, limiting the amount of light entering the eye. Conversely, when the surroundings are dim, the iris relaxes, allowing the pupil to expand and admit more light. This regulation of light is crucial for maintaining optimal vision and protecting the retina from damage due to excessive brightness.

While the pupil itself is essentially an opening controlled by the iris, it does not have the ability to adjust its size independently. The cornea is primarily responsible for focusing light as it enters the eye but does not regulate light intensity. The lens further assists with focusing light rays on the retina but does not influence the quantity of light entering the eye. Hence, the iris plays a vital role in controlling the overall light intake into the eye.