IGCSE Biology glossary
Iris
Biology editorial review is supported by the named review panel. Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.
Definition
The coloured circular structure in the eye that controls the size of the pupil, regulating how much light enters the eye. In bright light, circular muscles in the iris contract and radial muscles relax, making the pupil smaller. In dim light, radial muscles contract and circular muscles relax, making the pupil larger.
What to include in an exam answer
In the pupil reflex, name the two sets of muscles: circular (constrict the pupil) and radial (dilate the pupil). This is a reflex action and a common 4-mark structured question.
Example in context
Explain how the iris responds to bright light (3 marks): The circular muscles in the iris contract while the radial muscles relax. This makes the pupil smaller, reducing the amount of light entering the eye and protecting the retina.
Related syllabus topic
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