How does the eye detect light and change focus between near and distant objects, and how are sight defects corrected?
The structure and function of the parts of the eye, accommodation to focus near and distant objects, and the causes and correction of long-sightedness and short-sightedness.
A focused answer to the WJEC GCSE Biology section 2.5 topic on the eye, covering the structure and function of the cornea, iris, pupil, lens, retina and optic nerve, accommodation by the ciliary muscles and suspensory ligaments, and the causes and correction of long-sightedness and short-sightedness.
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What this dot point is asking
WJEC wants you to know the structure and function of the main parts of the eye, how the eye focuses light from near and distant objects (accommodation), and the causes and correction of long-sightedness and short-sightedness.
The structure and function of the eye
Light passes through several structures before forming an image.
| Part | Function |
|---|---|
| Cornea | Transparent front of the eye; refracts (bends) light as it enters |
| Iris | Coloured ring of muscle; controls the size of the pupil |
| Pupil | The hole that lets light into the eye |
| Lens | Refracts light to focus it onto the retina; changes shape |
| Retina | Layer of light receptors at the back of the eye |
| Optic nerve | Carries impulses from the retina to the brain |
The iris controls light entering the eye: in bright light it makes the pupil smaller to protect the retina; in dim light it makes the pupil larger to let more light in. This is itself a reflex.
How light forms an image
Light reflected from an object enters the eye and is refracted (bent) first by the cornea and then by the lens, so that it is focused onto the retina at the back of the eye. The retina contains light receptors that detect the light and produce electrical impulses. These impulses travel along the optic nerve to the brain, where the image is interpreted. The image formed on the retina is actually upside down, and the brain turns it the right way up.
The pupil reflex
The size of the pupil is controlled automatically by the iris, which is a ring of muscle. This is a reflex, so it happens without conscious thought.
- In bright light the iris muscles make the pupil smaller, reducing the light entering and protecting the delicate retina from damage.
- In dim light the iris muscles make the pupil larger, letting more light in so you can still see.
Accommodation: focusing near and far
Accommodation is the way the eye changes the shape of the lens to focus on objects at different distances. The lens is held by the suspensory ligaments, which are attached to the ciliary muscles.
- Near object: the ciliary muscles contract, the suspensory ligaments loosen, and the lens becomes fatter (more rounded), refracting light more strongly.
- Distant object: the ciliary muscles relax, the suspensory ligaments are pulled tight, and the lens is pulled thin, refracting light less.
Sight defects and their correction
Two common defects occur when the image does not form on the retina.
- Short-sightedness (myopia): distant objects look blurred because the image forms in front of the retina (the eyeball is too long or the lens too powerful). It is corrected with a concave (diverging) lens, which spreads light out before it enters the eye.
- Long-sightedness (hyperopia): near objects look blurred because the image forms behind the retina (the eyeball is too short or the lens too weak). It is corrected with a convex (converging) lens, which bends light inwards before it enters the eye.
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC style4 marksExplain how the eye changes focus to look at a near object after looking at a distant object (accommodation).Show worked answer →
A 4-mark explain question on accommodation.
To focus on a near object the ciliary muscles contract. This loosens the suspensory ligaments, so the lens is no longer pulled thin and becomes more rounded (fatter). A more rounded lens refracts (bends) the light more strongly, bringing the near object into focus on the retina.
Markers reward: ciliary muscles contract; suspensory ligaments loosen/slacken; lens becomes thicker/more rounded; refracts light more. Saying the lens "moves forward" instead of changing shape is a common error.
WJEC style3 marksA person cannot focus on distant objects clearly. Name this defect and describe how it is corrected.Show worked answer →
A 3-mark question on a sight defect.
The defect is short-sightedness (myopia). The image of a distant object forms in front of the retina because the eyeball is too long or the lens too powerful. It is corrected with a concave (diverging) lens, which spreads the light out before it enters the eye so the image forms on the retina.
Markers reward: short-sightedness; image forms in front of the retina; corrected with a concave/diverging lens. Confusing it with long-sightedness, or naming a convex lens, loses marks.
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Sources & how we know this
- WJEC GCSE Biology specification (from 2016) — WJEC (2016)