GCSE · Biology · AQA · Spec 8461

The eye (biology only)

Look across the room, then at your fingertip. Your eye just refocused, and nothing moved to do it: the lens inside changed shape. Here's how.

Biology · Anatomy

Follow the light through your eye

Showing 3 layers: Eye, Light path, Labels

Layers

Explore

tap a part, or turn the labels off ↓

All nine parts are labelled. Turn the labels off and try naming every part before you tap it.

Tap any part of the eye to see what it does. Then switch the labels off and test yourself.

Watch out: The pupil is a gap, not a structure — so if you're asked which structure controls the light getting in, the answer is the iris, not the pupil.

Near vs far: how the lens changes shape

Focusing on something NEARvsFocusing on something FAR AWAY

Picture the lens as a squashy ball held in the middle of a ring by strings. Pull the strings tight and the ball is stretched thin. Let them go slack and it springs back fatter. The ring is the ciliary muscle; the strings are the suspensory ligaments.

Focus

Ciliary muscles

Focusing on something NEAR

Contract

Focusing on something FAR AWAY

Relax

The insight

This is the one that catches people out. The muscles contract for NEAR things. The ciliary muscle is a ring, so contracting makes the ring smaller, closer to the lens, and it stops pulling on the ligaments.

Suspensory ligaments

Focusing on something NEAR

Loosen (go slack)

Focusing on something FAR AWAY

Pulled tight

Lens shape

Focusing on something NEAR

Thicker, more rounded

Focusing on something FAR AWAY

Pulled thin

How much the lens refracts light

Focusing on something NEAR

Strongly

Focusing on something FAR AWAY

Only slightly

From focusing to brightness

Why do your pupils get bigger in the dark?

You step out of a brightly lit kitchen into a dark garden. At first you can hardly see a thing, but a moment later the shapes of the garden appear. Your pupils have widened.

What made your pupils wider? Pick the idea closest to what you think right now.

I made them wider so I could see — I just didn't notice myself doing it.The circular muscles of the iris contracted and pulled the pupil open.The radial muscles of the iris contracted while the circular muscles relaxed, so the pupil widened.My lens got thicker so it could gather more of the dim light.

How sure are you?

I'm sureI think soI'm guessing

I made them wider so I could see — I just didn't notice myself doing it.

This is a common misconception

It feels like that, but nobody chooses it. Changes in pupil size are an automatic reflex response: they happen whether you're paying attention or not, and you don't decide them. The muscles of the iris do the work.

The circular muscles of the iris contracted and pulled the pupil open.

This is a common misconception

The circular muscles run round and round the pupil, like the drawstring on a bag. When a drawstring tightens, the opening gets SMALLER. So circular muscles contracting narrows the pupil — that's what happens in bright light. In dim light they relax.

The radial muscles of the iris contracted while the circular muscles relaxed, so the pupil widened.

This matches the evidence

Yes. The radial muscles run outwards from the pupil like the spokes of a wheel. When they contract, they pull the edge of the pupil outwards, and the relaxed circular muscles let it happen. A wider pupil lets more light into the eye, which helps you see in the dim light.

My lens got thicker so it could gather more of the dim light.

This is a common misconception

That mixes up two different jobs. Changing the lens's shape is how the eye focuses on near or distant things. Controlling how much light gets in is the iris's job: it changes the size of the pupil.

Dim light: radial muscles contract, circular muscles relax, pupil widens, more light gets in. Bright light is the reverse: circular muscles contract, radial muscles relax, the pupil narrows and less light gets in, which shields the retina from damage. Both happen automatically. It's a reflex.

When focusing goes wrong

Where do the rays meet?

View: 1 · Short sight. Showing 2 layers: Eye, Short sight

View

switch views — predict first ↑

Short sight (myopia): near things are sharp, but light from a distant object comes to a point IN FRONT of the retina and has spread out again by the time it arrives, so distant things look blurred. Predict before you switch: does the correcting lens need to bend the rays together MORE, or spread them apart a little?

Beyond glasses

Where does each treatment act?

Pick a treatment, then choose where it acts.

Still to sort

Adds an extra lens (0)

The eye isn't changed at all: an extra lens in the light's path does some of the refracting.

Where the line is: A contact lens touches the cornea but doesn't change it. Take the contact lens out and the eye is exactly as it was. Laser surgery changes the cornea itself.

Changes the shape of the cornea (0)

The front window of the eye is permanently reshaped, so it bends light by a different amount.

Replaces the eye's own lens (0)

The lens inside the eye is swapped for an artificial one.

5 of 5 still to sort.

Each of these treats short or long sight. Sort them by the part of the light's path they change.

Back to focusing: your turn

Explain focusing in your own words

You are watching a football match from the back of the stand. Your phone buzzes and you look down to read the message. Explain how your eye changes so that it can focus on the phone. [4 marks]

Your answer

0 words · your answer stays on this page and is not sent anywhere.

The mark scheme

The ciliary muscles contract. (1 mark)
The suspensory ligaments loosen (go slack). (1 mark)
The lens becomes thicker (more rounded). (1 mark)
The lens refracts the light more strongly, so it focuses on the retina. (1 mark)

Model answer

The phone is a near object. The ciliary muscles contract, so the suspensory ligaments loosen. With the ligaments slack, the lens becomes thicker. A thicker lens refracts the light more strongly, so the light from the phone is focused on the retina.

Each point earns its credit only if it's the right way round. 'The ciliary muscles relax' for a near object is wrong, not half right. Say what happens to the muscles, then the ligaments, then the lens, then the light.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Every part of your eye has one job on light's journey from the outside world to your brain.

What you need to know

Name the seven structures on a diagram of the eye (retina, optic nerve, sclera, cornea, iris, ciliary muscles, suspensory ligaments) and say how each one's structure suits its job.
Explain accommodation: how the lens changes shape to focus on a near object and on a distant object.
Explain how the iris changes the size of the pupil in dim light and in bright light, and that this is a reflex.
Interpret ray diagrams of short sight (myopia) and long sight (hyperopia), and show how spectacle lenses correct them.
Know the newer treatments: hard and soft contact lenses, laser surgery to reshape the cornea, and a replacement lens.

The big picture

The eye is a sense organ with receptors that respond to light intensity and colour. Light enters through the cornea, which refracts it, passes through the pupil, which the iris widens or narrows, and is focused by the lens onto the retina. The optic nerve carries impulses to the brain, and the tough sclera protects the whole eyeball. To focus, the ciliary muscles and suspensory ligaments change the shape of the lens (accommodation). Short and long sight happen when light doesn't focus on the retina, and they are corrected with lenses or newer treatments.

Key points

1The eye is a sense organ. Receptors in the retina respond to light intensity and colour.

2Cornea: clear and curved, so it lets light in and refracts it; it also protects the front of the eye.

3Iris: coloured ring of muscle that controls the size of the pupil, the gap light passes through.

4Retina: the lining at the back where light is focused and receptor cells detect it.

5Optic nerve: carries electrical impulses from the retina to the brain.

6Sclera: tough white outer coat that protects the eyeball.

7Accommodation means changing the shape of the lens to focus. Near: ciliary muscles contract, suspensory ligaments loosen, lens thicker, refracts strongly. Far: ciliary muscles relax, ligaments pulled tight, lens pulled thin, refracts only slightly.

8Dim light: radial muscles of the iris contract and circular muscles relax, so the pupil widens. Bright light: the reverse, so the pupil narrows and protects the retina. It is an automatic reflex.

9Myopia (short sight): rays meet in front of the retina; corrected with a concave lens. Hyperopia (long sight): rays would meet behind the retina; corrected with a convex lens.

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Worked example

Problem

Priya has been reading a book. She looks up at a bus at the far end of the street. Describe how her eyes change so that the bus is in focus.

⚠ Watch out

Saying the ciliary muscles contract to see something far away. It's the other way round: they contract for NEAR objects. Contracting makes the muscle ring smaller, which lets the ligaments go slack and the lens get thicker.

🧠

Memory hook

NEAR means the muscles WORK: the ciliary muscles contract, the ligaments go slack, and the lens gets fat. Far away is the lazy view: the muscles relax, the ligaments go tight, and the lens is pulled thin.

✓

Check yourself

Without looking back: trace light from outside your eye to your brain, naming each structure it meets and its job. Which two structures work together to change the lens's shape?

Flashcards

(14)

What does the cornea do?

It is clear and curved, so it lets light in and refracts (bends) it. It also protects the front of the eye.

What is the job of the iris?

The coloured ring of muscle that controls how much light gets in, by changing the size of the pupil.

Is the pupil a structure?

No. It is the opening in the middle of the iris that light passes through.

What does the lens do?

It refracts light to focus it on the retina. It is flexible, so its shape can change to focus on near or distant objects.

What does the retina contain, and what do they detect?

Receptor cells, which respond to light intensity (brightness) and colour.

What does the optic nerve do?

It carries electrical impulses from the retina to the brain, where the information is processed.

What is the sclera, and what is it for?

The tough white outer coat of the eye. It protects the eyeball.

What is accommodation?

Changing the shape of the lens to focus on near or distant objects.

Near object: what do the ciliary muscles, suspensory ligaments and lens do?

Ciliary muscles contract, suspensory ligaments loosen, lens becomes thicker and refracts light strongly.

What happens in the iris in dim light?

Radial muscles contract and circular muscles relax, so the pupil widens and more light gets in.

Why does the pupil narrow in bright light, and which iris muscles make it happen?

Less light gets in, which protects the retina from damage. The circular muscles contract and the radial muscles relax.

Myopia: where do the rays meet, and which lens corrects it?

In front of the retina (short sight). A concave (diverging) lens corrects it.

Hyperopia: where would the rays meet, and which lens corrects it?

Behind the retina (long sight). A convex (converging) lens corrects it.

Name three treatments for sight defects besides spectacles.

Hard or soft contact lenses, laser surgery to change the shape of the cornea, and a replacement lens in the eye.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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Why do your pupils get bigger in the dark?

Where does each treatment act?

Explain focusing in your own words

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