What part of the eye dilates or contracts based on the amount of light present?

The part of the eye that dilates or contracts based on the amount of light is the pupil. In bright light, the muscles in the iris contract, causing the pupil to constrict and reduce the amount of light entering the eye. Conversely, in low light conditions, the iris muscles relax, allowing the pupil to dilate and let in more light for better visibility. This process helps protect the retina and optimize vision in varying lighting conditions.