GCSE · Topic 5 Homeostasis and Response

AQA GCSE Biology: The Eye

A Triple award GCSE Biology revision guide to the eye: its structures, the pupil reflex, accommodation, and the correction of short and long sight.

This whole guide is Triple award only. The eye is the sense organ that detects light. This guide covers the parts of the eye and their jobs, how the pupil reflex adjusts to different light levels, how the lens focuses on near and distant objects, and how short and long sight are corrected. The theme throughout is bending light to focus it on the retina.

The structures of the eye

The eye has several parts, each with a role in detecting and focusing light.

Retina: contains light-sensitive receptor cells that detect light intensity and colour, and send impulses to the optic nerve.
Optic nerve: carries the impulses from the retina to the brain.
Sclera: the tough white outer layer that protects the eye.
Cornea: the transparent, curved front layer that refracts (bends) light into the eye.
Lens: a flexible, transparent disc that focuses light onto the retina.
Iris: a ring of muscle that controls the size of the pupil, regulating how much light enters.
Ciliary muscles and suspensory ligaments: together these control the shape of the lens.

A cross-section of the eye seen from the side, light entering from the left. At the front are the cornea, then the iris with the pupil as the opening through it, then the lens, held in place by the suspensory ligaments running to the ciliary muscle. The three layers of the wall are labelled from the outside in: the sclera, then the retina lining the inside at the back, with the optic nerve leaving from the back of the eye

Note the order light travels in: cornea, then pupil, then lens, then retina. The cornea does most of the refraction and the lens fine-tunes the focus, which is why only the lens needs to change shape.

The pupil reflex

The iris adjusts the pupil automatically to control how much light enters.

In dim light, the pupil dilates (widens): the radial muscles contract and the circular muscles relax, letting more light in.
In bright light, the pupil constricts (narrows): the circular muscles contract and the radial muscles relax, letting less light in and protecting the retina.

The iris seen from the front in two conditions. In dim light the radial muscles contract, pulling the iris outwards along the spokes, and the pupil is wide. In bright light the circular muscles contract, tightening the ring around the opening, and the pupil is small. The iris is the whole coloured ring of muscle in both

The two sets of muscles pull in different directions, which is the easiest way to remember them. The radial muscles run outwards like spokes, so contracting them pulls the opening wider; the circular muscles run around the pupil like a drawstring, so contracting them pulls it smaller.

Accommodation

Accommodation is how the eye focuses on objects at different distances by changing the shape of the lens.

To focus on a near object, the ciliary muscles contract, the suspensory ligaments loosen, and the lens becomes thicker (more curved), so it refracts the light more strongly.
To focus on a distant object, the ciliary muscles relax, the suspensory ligaments pull tight, and the lens is pulled thin (flatter), so it refracts the light less strongly.

In both cases, the aim is to focus the light exactly on the retina.

Eye defects and their correction

Two common defects happen when light is not focused on the retina.

Myopia (short-sightedness): distant objects cannot be focused, because light focuses in front of the retina (the eyeball is too long or the lens too thick). It is corrected with a concave lens, which spreads the light out before it enters the eye.
Hyperopia (long-sightedness): near objects cannot be focused, because light focuses behind the retina (the eyeball is too short or the lens too thin). It is corrected with a convex lens, which brings the light together more before it enters the eye.

Three eyes compared, each with a beam of light entering from the left. In normal vision the eyeball is a regular length and the lens a normal shape, and the focal point sits exactly on the retina. In myopia the eyeball is too long or the lens too thick, so the focal point falls in front of the retina, inside the eye. In hyperopia the eyeball is too short or the lens too thin, so the focal point falls behind the retina, outside the back of the eye

Compare where the yellow focal point sits in each eye. That single position is what the whole topic turns on: on the retina is normal, in front of it is myopia, behind it is hyperopia.

Note that each defect has two possible causes, and the diagram gives both. The eyeball being the wrong length is the one students usually write, but a lens that is too thick or too thin produces exactly the same result, and the mark scheme accepts either.

These defects can also be treated with contact lenses, laser eye surgery (which reshapes the cornea) or replacement lens surgery (an artificial lens is inserted).

Two photographs side by side. On the left a child sits at an eye test with a phoropter over their eyes, the instrument that holds different lenses in front of each eye in turn. On the right an opened box of daily contact lenses, each sealed in its own foil packet printed with the power of the lens

The instrument on the left holds lenses of different powers in front of each eye in turn until the letters come into focus, which measures how much refraction the eye is failing to do for itself. The packets on the right are printed PWR −3.00. A negative power means a concave lens, so these are lenses for myopia — the correction described above, made in a form that sits on the cornea instead of in front of it.

How this fits together

The eye and brain work together to sense the world. Another Triple-award part of coordinating the body is keeping its temperature steady, covered in the control of body temperature guide.