Final answer:
The iris is responsible for regulating pupil size and light entrance based on light levels.
Explanation:
The iris is the colored part of the eye that regulates pupil size and light entrance. It contains two sets of muscular fibers, one to constrict the pupil in bright light and another to dilate it in dim light. The iris is connected to muscles that control the pupil size based on light levels, ensuring the appropriate amount of light enters the eye.
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