Sense organs — the eye
Structure of the eye, and the pupil and accommodation reflexes.
Review these first
Learning objectives
What you need to be able to do
Teacher-mapped phrasing — check against the official Cambridge syllabus for exact wording.
- 14.3.1Identify the structures of the eye and describe the pupil reflex and accommodation.
6 minute read
Sense organs — the eye
The eye is a sense organ: a group of receptor cells responding to one stimulus, here light.
The structures
- Cornea — transparent front layer; does most of the refraction (bending) of light.
- Iris — the coloured ring of muscle controlling pupil size.
- Pupil — the hole letting light in (not a structure, but a gap).
- Lens — fine-focuses light onto the retina; its shape can change.
- Retina — contains light receptors: rods (dim light, no colour) and cones (colour, bright light).
- Fovea — the region of the retina with the highest concentration of cones, giving sharpest vision.
- Optic nerve — carries impulses to the brain.
- Blind spot — where the optic nerve leaves; no receptors, so no image forms there.
The pupil reflex
In bright light: circular muscles of the iris contract, radial muscles relax, the pupil constricts, and less light enters — protecting the retina from damage. In dim light: radial muscles contract, circular muscles relax, the pupil dilates, letting more light in. This is a reflex, so it is automatic and rapid. The two muscle types are antagonistic — one contracts as the other relaxes.
Accommodation — focusing
- Near object: ciliary muscles contract, suspensory ligaments slacken, and the lens becomes fatter (more curved), refracting light more strongly.
- Distant object: ciliary muscles relax, suspensory ligaments pull tight, and the lens becomes thinner, refracting light less.
The counter-intuitive part is that the ligaments slacken when the muscle contracts — because the ciliary muscle forms a ring, contracting it reduces the ring's diameter and releases the tension.
Think of it like this
The iris is a camera aperture and the lens is the focus ring. Accommodation is not moving the lens back and forth as a camera does — the eye changes the lens's *shape* instead.
Worked examples
Method, step by step
Describe what happens in the eye when a person moves from a dark room into bright sunlight.
- 1Light receptors detect the increase in light intensity.
- 2This is a reflex, so the response is automatic.
- 3The circular muscles of the iris contract and the radial muscles relax.
- 4The pupil constricts, so less light enters and the retina is protected from damage.
Circular iris muscles contract and radial muscles relax, constricting the pupil so less light enters and the retina is not damaged.
Common misconceptions
- Thinking the lens does most of the focusing. The **cornea** does most of the refraction; the lens fine-tunes it.
- Getting accommodation backwards. For a **near** object the lens becomes **fatter**, not thinner.
- Believing the pupil itself changes size actively. The pupil is a hole — the **iris muscles** change it.
In the exam
- For accommodation, state all three changes together: ciliary muscle, suspensory ligaments, and lens shape.
- For the pupil reflex, name both muscle types and say which contracts — "circular muscles contract" is the marking point, not just "the pupil gets smaller".