Edexcel IGCSE Biology · Spec 2.91-2.92
The Eye
The parts of the eye, how the eye focuses on near and far objects (accommodation), and the pupil reflex.
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The Eye
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Explained
The eye: two sets of muscles doing two different jobs
The eye contains two separate muscle systems, and mixing them up is the single most common mistake in this topic. The iris changes the size of the pupil to control how much light gets in. The ciliary muscles change the shape of the lens to focus. One controls quantity of light, the other controls focus, and they are not connected.
The parts, and what each does
- Cornea: the clear front layer, which does most of the bending of light.
- Pupil: the hole that lets light in. It is a gap, not a structure.
- Iris: the coloured ring of muscle that changes the size of the pupil.
- Lens: fine tunes the focus so the image lands on the retina.
- Retina: the light sensitive layer at the back where the image forms.
- Optic nerve: carries impulses from the retina to the brain.
The cornea does more of the focusing than the lens does. The lens adjusts, which is why it is the part that changes shape.
Accommodation: focusing on near and distant objects
For a near object, the ciliary muscles contract. This slackens the suspensory ligaments, and the lens becomes fatter and more curved, bending light more strongly.
For a distant object, the ciliary muscles relax. The suspensory ligaments pull tight, and the lens becomes thinner, bending light less.
The relationship is the part worth memorising, because it seems backwards: muscles contracting makes the ligaments looser, not tighter.
The pupil reflex
In bright light the circular muscles of the iris contract and the radial muscles relax, making the pupil smaller so less light enters. In dim light the radial muscles contract and the circular muscles relax, making the pupil larger.
It is a reflex, which means it happens without conscious thought, and its purpose is to protect the retina from damage by bright light.
What examiners say about this topic
Principal examiner reports for Edexcel International GCSE Biology are specific about the errors here. Reports record candidates confusing the circular muscles of the iris with the ciliary muscles around the lens, and warn directly against confusing focusing by the lens with the change in pupil diameter by the iris.
They also note a subtler loss of marks. Candidates who wrote that reducing the light prevents eye damage were not credited, because the answer needed to name retinal damage. Saying which structure is being protected is the difference between a general statement and a mark.
A credited response is described as stating that less light enters the eye, that the circular muscles of the iris contract, and that the radial muscles relax. All three parts, in one answer.
How to answer a pupil reflex question safely
Name the light condition, name both sets of iris muscles and what each does, say what happens to the pupil, and say what that achieves for the retina. Four steps, and the mark scheme usually has a point for each.
Spec 2.91-2.92
What you need to know
- Label the parts of the eye
- Explain how the eye focuses light
- Describe the pupil reflex
Active recall
Quick check
Answer each question before opening the answer.
How does the eye focus on a near object (accommodation)?
Ciliary muscles contract, suspensory ligaments slacken, and the lens becomes fatter/more convex.
What happens to the pupil in bright light?
Circular muscles of the iris contract and the pupil gets smaller (constricts) to protect the retina.
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