pupils

The pupil is the round opening in the center of the iris that regulates how much light reaches the retina. It appears black because light entering the eye is absorbed by surrounding tissues. The size is controlled by two iris muscles: the sphincter pupillae, which constricts the pupil, and the dilator pupillae, which dilates it. Parasympathetic input via the oculomotor nerve constricts the pupil in bright light or during near work, while sympathetic input dilates it in darkness or under arousal. The pupil participates in the pupillary light reflex and the near response. In healthy adults, typical diameters are about 2 to 4 millimeters in bright light and up to 6 to 8 millimeters in darkness. Abnormal pupil size or reactivity can indicate ocular or neurological problems, and drugs can alter size (for example, opioids cause miosis; anticholinergics or stimulants can cause mydriasis).

Pupil as student: The term pupil denotes a learner, usually a child or student in school. It comes from Latin pupillus, meaning a ward or dependent. In many places, pupil is used for primary education and sometimes general school-age learners, while student is more common for higher education or formal study. The plural form, pupils, refers to multiple learners or to the two openings in the eyes.