The pupil is the dark, round opening in the center of the iris. Its size changes depending on the amount of light entering the eye.

* In bright light, the pupil constricts (becomes smaller) to reduce the amount of light entering the eye and prevent damage to the retina.

* In dim light, the pupil dilates (becomes bigger) to allow more light to enter the eye, improving vision in low-light conditions.

The pupil's ability to change size is crucial for our vision to adapt to different lighting situations.