Cambridge O Level Biology · Syllabus 5090 · Coordination and Control
Pupil Reflex
What is Pupil Reflex?
A reflex action in which the size of the pupil changes in response to light intensity. In bright light the circular muscles of the iris contract and the radial muscles relax, making the pupil smaller so that less light reaches the retina. In dim light the radial muscles contract and the circular muscles relax, making the pupil larger so that more light enters.
This definition is part of the Coordination and Control chapter in Cambridge O Level Biology.
Last reviewed Syllabus 2026-2028