Final answer:
The iris adjusts the size of the pupil to control the amount of light entering the eye, ensuring optimal vision and protection for the retina.
Step-by-step explanation:
The structure in the eye that constricts or dilates to control the amount of light that enters the eye is called the iris. The iris contains muscles that adjust the size of the pupil, the opening through which light enters. In bright light conditions, the pupil narrows or constricts to limit the amount of light, and in dim light, the pupil widens or dilates to allow more light in. This adjustment mechanism is crucial for maintaining optimal vision and protecting the retina from damage due to excessive light exposure.