Final answer:
The iris is the structure that regulates the size of the pupil, the opening for light in the eye. It constricts the pupil in bright light and dilates it in low light. The cornea is a protective layer that also helps to focus light on the retina.
Step-by-step explanation:
The structure that regulates the size of the opening for light in the eye is known as the iris. The iris is responsible for controlling the amount of light that enters the eye through the pupil, which is the variable opening at the center of the iris. In bright light conditions, the iris causes the pupil to constrict, thereby reducing the amount of light entering the eye. Conversely, in dim lighting, the iris allows the pupil to dilate, increasing light entry to facilitate better vision.
Additionally, the cornea serves as a protective layer on the front of the eye, but it is also essential for focusing light as part of the eye's lens system. The focused light then passes through the pupil and is further focused by the lens to form a clear image on the light-sensitive retina. For objects at varying distances, the lens adjusts its power to ensure clear vision.