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Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.
Key concept
Light enters the eye through the cornea, passes through the pupil (controlled by the iris), is focused by the lens onto the retina, where light-sensitive receptor cells generate nerve impulses sent to the brain via the optic nerve. The pupil reflex adjusts pupil size to control the amount of light reaching the retina.
The eye is a key sense organ in the syllabus. Questions ask you to label the eye, explain the function of each part, and describe the pupil reflex. Focus on linking each structure to a specific function.
Key structures and their functions
Structure
Function
Cornea
Transparent front part of the eye that refracts (bends) light entering the eye
Iris
Coloured part with muscles that control the size of the pupil
Pupil
Opening in the centre of the iris that allows light to enter
Lens
Transparent, flexible structure that focuses light onto the retina by changing shape (accommodation)
Retina
Layer of light-sensitive receptor cells (rods and cones) at the back of the eye
Optic nerve
Carries nerve impulses from the retina to the brain
Ciliary muscles and suspensory ligaments
Control the shape of the lens for focusing on near or distant objects
The pupil reflex
In bright light, the circular muscles of the iris contract and the radial muscles relax. This makes the pupil smaller, reducing the amount of light entering the eye and protecting the retina from damage.
In dim light, the radial muscles contract and the circular muscles relax. The pupil becomes larger, allowing more light to enter so the retina can still detect images. This is a reflex action: it is automatic and involuntary.
Accommodation (Supplement)
Accommodation is the ability of the eye to focus on objects at different distances by changing the shape of the lens.
Near objects: ciliary muscles contract, suspensory ligaments become slack, and the lens becomes thicker and more curved. This increases refraction to focus the image on the retina.
Distant objects: ciliary muscles relax, suspensory ligaments become taut and pull the lens into a thinner, less curved shape. Less refraction is needed for distant light rays.
Try a focused question (3 marks)
Describe how the eye adjusts to a sudden increase in light intensity.
Show the answer and marking guidance
The circular muscles of the iris contract.
The radial muscles of the iris relax.
The pupil becomes smaller, reducing the amount of light entering the eye to protect the retina.
Original Cambridge-style practice written for this site. It is not an official Cambridge past-paper question.
Common mix-ups to avoid
Check these points against your own answer before moving on.
Confusing circular and radial muscles: circular contract to make the pupil smaller in bright light.
Saying the pupil is a structure that contracts: the pupil is a hole; the iris muscles contract.
Mixing up ciliary muscle contraction with suspensory ligament tightening: they have opposite states.
Describing accommodation as the pupil reflex: accommodation changes lens shape; the pupil reflex changes pupil size.