The eye: GCSE labelled diagram

A GCSE eye diagram shows a side view of the eyeball with seven structures you need to label: the cornea, iris, ciliary muscles, suspensory ligaments, sclera, retina and optic nerve, plus the lens and pupil. Light is refracted by the cornea and lens and focused onto the retina, where receptor cells turn it into electrical impulses for the optic nerve to carry to the brain.

The labelled eye diagram

AQA GCSE Biology lists the structures you must be able to identify on a diagram and link to their jobs: retina, optic nerve, sclera, cornea, iris, ciliary muscles and suspensory ligaments. The eye is a sense organ, with receptors sensitive to light intensity and colour.

The parts of the eye and what they do

The retina has two kinds of receptor: rods work in dim light but do not detect colour, and cones detect colour but need brighter light. Where the optic nerve leaves the eye there are no receptors, which gives each eye a blind spot.

How the eye focuses: accommodation

Accommodation is changing the shape of the lens to focus on near or distant objects. Light from a near object is spreading out more when it reaches the eye, so the lens has to bend it more strongly.

The counter-intuitive part is that the lens is pulled thin when the muscles relax. The relaxed ring of ciliary muscle is wide, so it pulls the ligaments tight, and they stretch the lens. When the muscles contract, the ring gets smaller, the ligaments go slack, and the lens springs back to a fatter shape.

Adapting to dim and bright light

AQA also asks how the eye adapts to dim light. The iris is muscle: in dim light it dilates (widens) the pupil to let more light reach the retina, and in bright light it constricts (narrows) the pupil, which protects the retina. This is a reflex, so it happens automatically.

Short sight and long sight

In the two common defects, rays of light do not focus on the retina, so the image is blurred.

Myopia (short sight)

Distant objects look blurred. The eyeball is too long from front to back, or the cornea or lens bends light too much, so light focuses in front of the retina.

Hyperopia (long sight)

Nearby objects look blurred. The eyeball is too short, or the cornea or lens does not bend light enough, so light would focus behind the retina.

Correcting the defects

AQA also names newer treatments: hard and soft contact lenses, laser surgery to change the shape of the cornea, and replacing the lens inside the eye. Try lenses on a beam of light in the Ray Optics Simulator to see how converging and diverging lenses move the focus.

How to draw and label an eye diagram in the exam

1. Use a ruler for label lines, and make each line touch the structure it names.
2. Keep labels outside the drawing, and do not cross label lines.
3. Point to the right place: the retina is the inner layer at the back, the sclera is the white outer layer, and the ciliary muscles sit at the edge, joined to the lens by the ligaments.
4. For ray diagrams, draw straight lines with arrows showing the direction light travels, and show exactly where the rays meet.

Common mistakes

• Getting accommodation backwards. Near object: muscles contract, ligaments slacken, lens fatter.
• Saying the pupil is a structure that moves. The pupil is a hole; the iris changes its size.
• Mixing up the lens types. Short sight needs a concave lens; long sight needs a convex lens.
• Saying only the lens focuses light. The cornea does much of the bending; the lens adjusts the focus.
• Labelling the sclera as the retina. The sclera is outside; the retina is the inner layer where the image forms.

Practise with a short timed paper

The eye is in AQA GCSE Biology (4.5.2.3), but not in Combined Science. The daily GCSE Biology Higher paper is a short timed paper of 8 questions, with a new one every day. For the whole topic, see homeostasis and response.

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