The Eye — Focusing, and the Pupil Reflex

What this lesson covers in the specification

Incandio is aligned to this specification. It is not published by, endorsed by or affiliated with Pearson, and it reproduces none of Pearson's wording — the statement numbers are given so you can check every lesson against your own copy.

• 2.91 — The structure and function of the eye as a receptor
• 2.92 — How the eye focuses on near and distant objects and responds to light intensity

Specification section 2(j) Co-ordination and response — also taught in ADH and the Water Balance of the Blood, Homeostasis and the Control of Body Temperature, Plant Responses, Tropisms and Auxin, The Nervous System, and the Two Ways a Body Sends a Message.

1 · Understand it

No exam language yet. The only question this section answers is: do I actually understand what is happening?

Only one part of the eye actually detects anything. The retina, lining the inside of the back of the eyeball, contains the light receptors; everything in front of it exists to deliver a sharp, correctly bright image onto it. Once you hold that idea, the rest of the eye stops being a list of parts to memorise and becomes a set of solutions to two problems: getting the image in focus, and getting the amount of light right.

What each part is for

1. CORNEA — the transparent layer at the front. It refracts (bends) the light, and does most of the bending. Its shape is fixed, so it cannot adjust.
2. IRIS — the coloured ring of muscle. It changes the size of the pupil, and so controls how much light gets in.
3. PUPIL — not a structure at all, but the hole in the middle of the iris that the light passes through.
4. LENS — transparent and elastic, and able to change shape. It provides the fine adjustment that the fixed cornea cannot.
5. CILIARY MUSCLES and SUSPENSORY LIGAMENTS — the ring of muscle around the lens, and the fibres joining that ring to the edge of the lens. Together they control the lens's shape.
6. RETINA — contains the light receptors, which convert light into electrical impulses.
7. OPTIC NERVE — carries those impulses to the brain.

Now the focusing. Light from a distant object arrives at your eye almost parallel, so it needs only a little bending to be brought to a point on the retina. Light from something close arrives spreading outwards, and needs a great deal more bending in the same short distance. The cornea cannot change, so the lens has to make up the difference: fatter and more curved for a near object, thinner and flatter for a distant one.

Think of it like a trampoline held by a ring of springs

Picture the lens as a trampoline mat and the suspensory ligaments as the springs joining it to a circular frame — the ciliary muscle. Left to itself the mat is naturally domed and fat; the springs are what stretch it out flat. Now make the frame SMALLER. Every spring goes slack, stops pulling, and the mat springs back into its natural fat shape. Make the frame bigger and the springs pull tight and stretch it flat again. That is the whole mechanism, and it explains the step everyone gets backwards: the ciliary muscle is a ring, so CONTRACTING it makes the ring smaller, which SLACKENS the ligaments and lets the lens become fat.

Focusing on a NEAR object

1. The light rays entering the eye are diverging, so they need a lot of refraction to be brought to a point on the retina.
2. The ciliary muscles CONTRACT, so the ring they form becomes smaller in diameter.
3. The suspensory ligaments therefore SLACKEN and stop pulling on the edge of the lens.
4. The lens is elastic, so it springs back into its natural shape: thicker and more curved.
5. A fatter lens refracts the light more strongly, so the rays are brought to a point exactly on the retina and the image is sharp.

Focusing on a DISTANT object

1. The light rays entering the eye are almost parallel, so they need only a little refraction.
2. The ciliary muscles RELAX, so the ring they form becomes larger in diameter.
3. The suspensory ligaments are pulled TIGHT, and pull outwards on the edge of the lens.
4. The lens is stretched thinner and flatter.
5. A thinner lens refracts the light less, so the nearly parallel rays are brought to a point on the retina rather than in front of it.

There is a consequence of that elasticity worth knowing. The lens springs into its fat shape by itself, using nothing but its own springiness — the muscle only lets it. As people get older the lens gradually becomes stiffer, so even with the ciliary muscles fully contracted and the ligaments completely slack it can no longer round up as much as it used to. Distant vision is unaffected, because that needs a flat lens and stretching still works. It is close work that goes first, which is why reading glasses are so nearly universal after about the age of forty-five.

The second adjustment is about brightness rather than sharpness, and it is a reflex — automatic, rapid and protective. In bright light the CIRCULAR muscles of the iris contract and the radial muscles relax, so the pupil becomes smaller and less light enters, protecting the receptors of the retina from damage. In dim light the RADIAL muscles contract and the circular ones relax, so the pupil widens and as much light as possible is let in. The two sets of muscles work against each other — one closes the hole, the other opens it — which is the only way a muscle can ever produce movement in both directions, since a muscle can pull but never push.

One thing to be exact about, because it is a mark: the pupil does not do anything. It is a hole. Saying 'the pupil contracts' describes the outcome, not the mechanism — what contracts is a set of muscles in the iris, and the pupil is simply the space they leave. And keep the two adjustments apart in your head: the iris and pupil control the QUANTITY of light, and the lens controls where that light is brought to a focus. Neither one helps with the other's job.

2 · Grade 9 Notes

A different job from the section above. You have already understood it; this is the precise set of things to LEARN — definitions to reproduce word for word, processes in order, equations with units, and the answers that score full marks.

Learn this definition · Accommodation

The change in the shape of the lens that allows the eye to focus light from objects at different distances onto the retina.

Learn this definition · Refraction

The change in direction of light as it passes from one transparent medium into another of different density — in the eye, at the cornea and at the lens.

Statement 2.91 — the parts of the eye and the function of each

Cornea

Transparent front layer; refracts light, and does most of the refraction. Its shape cannot change.

Iris

Ring of muscle containing circular and radial muscles; alters the size of the pupil and so controls how much light enters.

Pupil

The hole in the centre of the iris through which light passes. It is a gap, not a structure.

Lens

Transparent and elastic; changes shape to make the fine adjustment to focusing.

Ciliary muscles

A ring of muscle around the lens; contracting makes the ring smaller and slackens the suspensory ligaments.

Suspensory ligaments

Join the ciliary muscles to the edge of the lens; when tight they pull the lens thin.

Retina

Contains the light receptors, which convert light into electrical impulses.

Optic nerve

Carries the impulses from the retina to the brain.

Accommodation — near against distant, the sequence in both directions

The pupil reflex — bright against dim, and which muscles do it

How much light gets in — the calculation the data question uses

1. Light passes through the pupil, so the amount entering depends on the AREA of the pupil, not on its diameter.
2. Area of a circle = πr², and r is half the diameter — so area is proportional to (diameter)².
3. To compare two pupil sizes, square the ratio of the diameters: doubling the diameter lets in 2² = 4 times as much light.
4. A pupil going from 2 mm to 8 mm is 4 times wider, so it admits 4² = 16 times as much light.

The two adjustments, which are constantly confused

A. THE LENS AND CILIARY MUSCLES control FOCUS — where the light is brought to a point. They do nothing about brightness.

B. THE IRIS AND PUPIL control the QUANTITY of light entering. They do nothing about focus.

Not this: The ciliary muscles contract to pull the lens thin.

This: They are a RING. Contracting makes the ring smaller, which slackens the ligaments and lets the elastic lens spring back to a fatter shape. It is relaxing that lets the ligaments pull it thin.

Model answer [4 marks]

Describe how the eye changes to focus on a near object. [4]

The ciliary muscles contract, so the ring of muscle becomes smaller in diameter. The suspensory ligaments therefore slacken and stop pulling on the lens. The elastic lens becomes thicker and more curved. It refracts the light more strongly, so the diverging rays from the near object are brought to a focus on the retina.

Model answer [4 marks]

Explain how the eye responds when a person walks from a dark room into bright sunlight. [4]

The light receptors in the retina detect the increase in light intensity. This is a reflex: the circular muscles of the iris contract and the radial muscles relax. The pupil therefore becomes smaller, so less light enters the eye. This protects the light receptors of the retina from being damaged by the intense light.

Model answer [2 marks]

Explain why the iris contains two sets of muscles rather than one. [2]

A muscle can only pull; it cannot push, so it cannot reverse its own effect. The circular and radial muscles are antagonistic — the circular muscles contracting makes the pupil smaller, and the radial muscles contracting makes it larger — so two sets are needed to move the pupil in both directions.

Mark-losing trap. The pupil is a HOLE. Muscles in the iris change its size; the pupil itself contracts nothing.

Mark-losing trap. Ciliary muscles CONTRACT for a near object. That slackens the ligaments — it does not pull the lens.

Mark-losing trap. Say 'the light receptors in the retina', not 'the eye detects the light'. The eye is the sense organ, not the receptor.

Mark-losing trap. The cornea does most of the refraction; the lens does the adjusting. A question about FOCUSING is about the lens.

Mark-losing trap. The pupil reflex protects the retina in bright light. 'So you can see better' is not the reason for constriction.

3 · Prove it — the five questions

The five questions climb Grade 6 → Grade 7 → Grade 8 → Grade 9 → Grade 9 challenge, and are marked inside Incandio on your own device, by rule, with an authored diagnosis of the mistake you actually made. The mark schemes stay in the app so that the practice is worth doing; the questions themselves are here.

1. Grade 6 · State [1 mark] — Which part of the eye contains the light receptors?
2. Grade 7 · Describe [2 marks] — A person walks out of a cinema into bright sunlight. Describe what happens in their iris, and what effect it has.
3. Grade 8 · Explain [5 marks] — A student looks up from a distant blackboard to read a book held close to their face. Explain how their eye changes to focus on the book. Select every statement that belongs in a full-mark explanation.
4. Grade 9 · Calculate [3 marks] — In dim light a person's pupil has a diameter of 8.0 mm. In bright light it has a diameter of 2.0 mm. Calculate how many times more light enters the eye in the dim conditions than in the bright ones.
5. 9+ · Analyse [5 marks] — A person of fifty finds they can still see distant road signs perfectly clearly, but can no longer focus on small print unless they hold it at arm's length. Tests show their ciliary muscles contract normally and their pupil reflex is unaffected. Select every statement that belongs in a full-mark explanation.

The people behind this science

Two ways into the same idea — the one who worked out what the eye actually does, and the one who explained why a curved transparent thing focuses. Inside Incandio each of them answers knowing exactly which lesson you have just finished.

Johannes Kepler — the one who worked out what the eye actually does

Before Kepler, nobody had a correct account of what happens inside an eye — the leading view placed vision in the lens itself, because an image on the retina would have to be upside down and that seemed impossible. In 1604 Kepler treated the eye as an optical instrument like any other, traced the rays through the cornea and lens, and showed that they form a real, inverted image on the retina, whose job is simply to receive it. He also said plainly that what happens afterwards was not his problem but the physiologists', which is the honest boundary this lesson sits on.

• “How did you work out that the image on the retina is upside down?”
• “Why did that conclusion trouble people so much?”
• “What convinced you to treat the eye as an instrument rather than as an organ?”
• “What did you decide was NOT your problem to solve, and why?”
• “How much of the bending did you think the cornea was doing?”

Isaac Newton — the one who explained why a curved transparent thing focuses

This lesson keeps saying that the cornea and lens refract light, and Newton is the reason that word can be used with confidence. His work on refraction established how light changes direction when it passes into a denser transparent medium, and that white light is a mixture of colours refracted by different amounts — which is why a simple lens can never focus every colour at the same point. He also investigated his own vision by pressing a blunt needle behind his eyeball to see what coloured circles appeared, which is either the most committed or the most alarming experiment in this course.

• “What happens to light when it passes into something denser, like the eye?”
• “Why can no single lens focus every colour at exactly the same place?”
• “What did you see when you pressed behind your own eye, and what did it tell you?”
• “Is the eye a good optical instrument or a poor one, in your judgement?”

Carry on through the course

• Previous lesson: Synapses and the Reflex Arc
• Next lesson: The Hormones — Sources, Roles and Effects
• Levels of Organisation — From organelle to organism — the five-step ladder that every later topic in biology is built on.
• Comparing Cells, and Stem Cells — What plant and animal cells share and where they differ — then why a cell specialising is one of the most powerful ideas in biology.
• Carbohydrates, Proteins and Lipids — The three big food molecules — what they are made of, what they are built from, and how to test for them.
• Enzymes: Catalysts With a Shape — Why a protein with a hole in it speeds up a reaction, and why heat destroys that ability permanently.
• Enzymes and pH — Why acid ruins one enzyme and is the only condition another will work in — and how to find an enzyme's best pH by experiment.
• All of Biology · Biology key terms · required practicals · Incandio Science · The 4BI1 specification