Which structure regulates the amount of light entering the eye?

The structure that regulates the amount of light entering the eye is the pupil. The pupil is an opening in the center of the iris, which is the colored part of the eye. It functions much like the aperture of a camera, adjusting in size to control the amount of light that reaches the retina for proper vision. When light levels are high, the muscles in the iris constrict the pupil to reduce the amount of light entering the eye, helping to protect the retina from excessive brightness. Conversely, in low light conditions, the pupil dilates to allow more light in, thereby enhancing vision in dim environments.

The lens, while critical for focusing light onto the retina, does not regulate light intensity—its primary function is to change shape to focus light rays properly for clear vision. The cornea also helps in focusing but is not directly involved in regulating light entry. The retina is the light-sensitive layer at the back of the eye that detects the incoming light and sends visual signals to the brain, but it does not play a role in controlling how much light enters the eye. Thus, the pupil is the correct answer as it actively adjusts to varying light conditions to optimize visual performance.