What is the role of the iris?

• A. Produces aqueous humor
• B. Regulates pupil diameter to control light entry
• C. Focuses light on the retina
• D. Transmits light to the optic nerve

Answer: (B)

The iris mainly adjusts how much light reaches the retina by changing the size of the pupil. It acts as a tiny diaphragm with two smooth muscle groups: the sphincter pupillae, which constricts the pupil in bright light, and the dilator pupillae, which widens it in dim light. This dynamic control is driven by the autonomic nervous system—parasympathetic signals tighten the sphincter to reduce light entry, while sympathetic signals activate the dilator to allow more light in. By modulating pupil diameter, the iris helps protect the retina from glare and improves visual acuity across lighting conditions. The iris doesn’t produce aqueous humor (that’s the ciliary body), nor focus light (that’s the cornea and lens), nor transmit light to the optic nerve (the retina does that).