Chapter 3 of 8

All 8 chapters
  1. 1The nervous system, readunderstood
  2. 2Reflexes, readunderstood
  3. 3The eye, readunderstood
  4. 4Hormones, readunderstood
  5. 5Blood sugar and diabetes, readunderstood
  6. 6Temperature and water balance, readunderstood
  7. 7Hormones and the menstrual cycle, readunderstood
  8. 8Plant responses, readunderstood

S4.5.3 Chapter 3

The eye

How does your eye let in the right amount of light, and how does it see near things and far things clearly?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

  • You picked A. Read on. At the end, the chapter comes back to your guess.

  • You picked B. Read on. At the end, the chapter comes back to your guess.

  • You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

  • Label the eye and give the job of each part
  • Explain how the pupil changes in bright and dim light
  • Explain how the lens focuses on near and far objects

You see an object when light from it enters your eye. The eye makes a small picture of the object on a layer at the back. That layer sends impulses to the brain, and the brain makes sense of them. The picture on the back of the eye is upside down. The brain turns it the right way up.

Draw it first

What the parts of the eye do

  1. Three pictures of the eye cut across, front on the left, one after another. First the labels are cornea, pupil, lens and retina. Then they are iris, ciliary muscle and suspensory ligaments. Last they are retina, optic nerve and sclera.corneapupillensretina

    1 Light enters through the cornea and the pupil. The cornea and the lens bend it so that it makes an image on the retina.

  2. Three pictures of the eye cut across, front on the left, one after another. First the labels are cornea, pupil, lens and retina. Then they are iris, ciliary muscle and suspensory ligaments. Last they are retina, optic nerve and sclera.ciliarymuscleirissuspensoryligaments

    2 The iris changes the size of the pupil. The ciliary muscle and the suspensory ligaments change the shape of the lens.

  3. Three pictures of the eye cut across, front on the left, one after another. First the labels are cornea, pupil, lens and retina. Then they are iris, ciliary muscle and suspensory ligaments. Last they are retina, optic nerve and sclera.scleraopticnerveretina

    3 The retina turns light into impulses. The optic nerve carries them to the brain. The sclera is the tough outer wall.

Now each part and its job. The cornea is the clear window at the front. It bends light more than any other part. The iris is the coloured ring. It has muscles that change the size of the pupil, the hole in the middle. The pupil looks black because the light that goes in is not sent back out.

Behind the pupil is the lens. It bends the light a little more, and it can change its shape to give a sharp image. The ciliary muscle is a ring of muscle round the lens. Suspensory ligaments are fibres that join the muscle to the lens. They hold the lens in place.

The retina lines the back of the eye. It holds the receptors for light. Rods work in dim light and do not see colour. Cones need bright light and see colour. The optic nerve carries impulses from the retina to the brain. The tough white wall of the eye is the sclera.

The eye cut across, with the front on the left and the back on the right. The labels are cornea, iris, pupil, lens, ciliary muscle, suspensory ligaments, retina, optic nerve and sclera.ciliarymuscleiriscorneapupillenssuspensoryligamentsscleraopticnerveretina
The eye cut across, with the front on the left. The picture is a model. A real eye also holds clear jelly and fluid that keep its shape.

Now the pupil. Too much light can harm the retina, and too little gives a poor picture. The iris has two sets of muscles. Circular muscles go round the pupil like the rings of a target. Radial muscles run outwards like the spokes of a wheel.

In bright light, the circular muscles contract and the radial muscles relax. The pupil becomes smaller, so less light gets in. In dim light, it is the other way round. The radial muscles contract and the circular muscles relax, so the pupil becomes wider.

The front of the eye in bright light. The pupil is small. The labels are circular muscle contracted, radial muscle relaxed, iris, pupil and bright light.bright lightcircularmusclecontractedradialmusclerelaxedpupiliris
Bright light. The circular muscles are contracted, so the pupil is small.
The front of the eye in dim light. The pupil is wide. The labels are circular muscle relaxed, radial muscle contracted, iris, pupil and dim light.dim lightcircularmusclerelaxedradialmusclecontractedpupiliris
Dim light. The radial muscles are contracted, so the pupil is wide.

This is a reflex. Receptors in the retina sense the light. Impulses go along the optic nerve to the brain, and motor neurones carry the answer back to the muscles of the iris. They are the effectors. You cannot decide to do it.

How much difference does it make? Look at the typical sizes in the table. The amount of light that gets in depends on the area of the pupil. The area of a circle is π × r × r, where r is the radius, half the width.

A table with the columns Part and Width. The rows are iris 12 mm, pupil, bright 3 mm, and pupil, dim 7 mm. These are typical widths.PartWidthiris12 mmpupil, bright3 mmpupil, dim7 mm
Typical widths. They differ a little from person to person.

Worked example

What is the area of a pupil that is 3 mm wide, in bright light?

  1. The radius is half the width: 3 ÷ 2 = 1.5 mm.
  2. Area = π × 1.5 × 1.5 = 7.07 mm².

Answer: 7.07 mm²

Worked example

A pupil is 3 mm wide in bright light and 7 mm wide in dim light. How many times bigger is the area in dim light?

  1. In dim light the radius is 7 ÷ 2 = 3.5 mm.
  2. Its area is π × 3.5 × 3.5 = 38.48 mm².
  3. In bright light the area is 7.07 mm², from the last example.
  4. Divide: 38.48 ÷ 7.07 = 5.44, so the area is about 5.4 times bigger.

Answer: 5.4 times

Now focus. Light from an object must meet at one point on the retina, or the picture is blurred. The cornea bends light most. The lens does the fine work, and it can change its shape. A fatter lens bends light more. A thinner lens bends it less.

Light from a near object spreads out more as it enters the eye. It needs more bending, so the lens must be fat. The ciliary muscle contracts, and its ring gets smaller. The suspensory ligaments slacken. The lens is elastic, so it springs into a fatter shape.

Light from a near object enters a thick lens and meets at one point on the retina. The labels are ciliary muscle contracted, thick lens, retina and near object.ciliary musclecontractedthicklensretinanear object
A near object. The ciliary muscle is contracted and the lens is thick. The rays meet on the retina. The distances are drawn closer than real ones, to fit the page.

Light from a far object is almost parallel when it enters the eye. It needs less bending. The ciliary muscle relaxes and its ring gets larger. The suspensory ligaments pull tight and stretch the lens thin.

Light from a far object enters a thin lens and meets at one point on the retina. The labels are ciliary muscle relaxed, thin lens, retina and far object.ciliary musclerelaxedthinlensretinafar object
A far object. The ciliary muscle is relaxed and the lens is thin. The rays meet on the retina again.
A table with the columns Part, Near and Far. Muscle: contracts for near, relaxes for far. Ligament: slack for near, tight for far. Lens: fat for near, thin for far.PartNearFarmusclecontractsrelaxesligamentslacktightlensfatthin
Focusing in one table. The muscle is the ciliary muscle. The ligament is the set of suspensory ligaments.

Some eyes cannot focus well. In short sight, far things look blurred and near things are clear. The eyeball is too long, or the lens is too strong, so light from far things meets in front of the retina. In long sight, near things look blurred and far things are clear. The eyeball is too short, or the lens too weak, so light from near things would meet behind the retina.

A table with the columns Sight, Blurred and Lens. Short sight: far things, spreads. Long sight: near things, bends.SightBlurredLensshortfar thingsspreadslongnear thingsbends
Short and long sight. Glasses put a lens in front of the eye, so that the image falls on the retina. The lens that spreads light helps short sight. The lens that bends light inwards helps long sight.

Anyone who sees badly, or who has pain in the eye, should see a health worker or an eye specialist. They can test the eyes and say which glasses are right.

Try it

Watch a pupil change size

You need: a friend, a mirror, if you work alone, a room with ordinary daylight or room light, a clock or a way of counting seconds.

Be safe: Use only the ordinary light of the room. Never look at the Sun. Never shine a torch or a lamp at anyone's eyes.

  1. Sit in an ordinary lit room, not in direct sunlight. Your friend sits facing you.
  2. Cover your eyes with your palms for 30 seconds. Do not press on them. This makes the pupils wide.
  3. Take your hands away. Your friend watches your pupils for the next few seconds. Or look in a mirror.
  4. Write down what you see. Then swap places.

What you will see: Straight after you uncover your eyes, the pupils are wide. Within a second or two they get smaller. The muscles of the iris do this as a reflex, in answer to the light that enters.

If you cannot try it, this is what happens.
A table with the columns When and Pupil. Hands just off: wide. Seconds later: smaller.WhenPupilhands just offwideseconds latersmaller

A wrong idea many people have

In people, the eye focuses by moving the lens towards or away from the retina, like a camera.

Why it feels right: A camera focuses by moving its lens, and the eye is often compared with a camera.

The test that breaks it: The lens of the eye is held in place by the suspensory ligaments. What changes is its shape. The ciliary muscle makes it fatter, and the ligaments make it thinner.

What is true: The lens does not move. The ciliary muscle and the ligaments change its shape, so that it bends light more or less.

Key words

cornea
the clear window at the front of the eye. It bends light most.
iris
the coloured ring with muscles that change the size of the pupil.
pupil
the hole in the middle of the iris, where light goes in.
lens
the part behind the pupil that changes shape to focus light.
retina
the layer at the back of the eye with receptors for light.
optic nerve
the nerve that carries impulses from the retina to the brain.
ciliary muscle
a ring of muscle round the lens that changes the shape of the lens.
suspensory ligaments
fibres that join the ciliary muscle to the lens and hold it in place.

Back to your guess

The answer is C: The pupil gets wider, so that more light can get in.

In dim light the muscles of the iris make the pupil wider, so more light reaches the retina. In bright light they make it smaller, which protects the retina.

Check yourself

Try each one before you look. A wrong answer tells you where to go next.

  1. Which part of the eye has receptors that detect light?

    Tap your answer.

    1. Yes, that is right. The retina lines the back of the eye. Its receptors, the rods and cones, detect the light and start impulses in the optic nerve.

    2. Not this one. The cornea is the clear window at the front of the eye. It bends light, but it has no receptors. Try another.

    3. Not this one. The iris is the coloured ring. Its muscles change the size of the pupil. Try another.

    4. Not this one. The lens bends light and changes shape to focus. The receptors are in the retina, at the back. Try another.

    See the answer

    The retina

    The retina lines the back of the eye. Its receptors, the rods and cones, detect the light and start impulses in the optic nerve.

    • If you chose B: The cornea is the clear window at the front of the eye. It bends light, but it has no receptors.
    • If you chose C: The iris is the coloured ring. Its muscles change the size of the pupil.
    • If you chose D: The lens bends light and changes shape to focus. The receptors are in the retina, at the back.

  2. Ana walks out into bright light. What happens in her iris?

    Tap your answer.

    1. Not this one. Radial muscles contract in dim light, and that widens the pupil. In bright light the pupil gets smaller. Try another.

    2. Yes, that is right. In bright light the circular muscles contract and the radial muscles relax. The pupil gets smaller and protects the retina.

    3. Not this one. When the circular muscles contract, they pull the pupil smaller, not wider. Try another.

    4. Not this one. The radial muscles run outwards like spokes. When they contract they pull the pupil wider. Try another.

    See the answer

    The circular muscles contract and the pupil gets smaller.

    In bright light the circular muscles contract and the radial muscles relax. The pupil gets smaller and protects the retina.

    • If you chose A: Radial muscles contract in dim light, and that widens the pupil. In bright light the pupil gets smaller.
    • If you chose C: When the circular muscles contract, they pull the pupil smaller, not wider.
    • If you chose D: The radial muscles run outwards like spokes. When they contract they pull the pupil wider.

  3. Why does the pupil get wider in dim light?

    Tap your answer.

    1. Not this one. The pupil gets smaller to protect the retina in bright light. In dim light the pupil must let in all the light it can. Try another.

    2. Not this one. The size of the pupil does not change the lens. The ciliary muscle changes the lens. Try another.

    3. Yes, that is right. A wider pupil has a larger area, so more light enters. This helps the retina to work in dim light.

    See the answer

    To let more light reach the retina

    A wider pupil has a larger area, so more light enters. This helps the retina to work in dim light.

    • If you chose A: The pupil gets smaller to protect the retina in bright light. In dim light the pupil must let in all the light it can.
    • If you chose B: The size of the pupil does not change the lens. The ciliary muscle changes the lens.

  4. Yusuf has been looking at a far hill. Now he looks at a book held close to his face. What happens in his eye?

    Tap your answer.

    1. Yes, that is right. Light from a near object needs more bending. The lens must be fatter. The contracted ciliary muscle lets the ligaments go slack, and the elastic lens becomes fatter.

    2. Not this one. When the ciliary muscle contracts, the ligaments slacken, not tighten. Then the lens springs fatter. Try another.

    3. Not this one. That is what happens when you look at something far away. A near object needs a fatter lens. Try another.

    See the answer

    The ciliary muscle contracts, the ligaments slacken and the lens gets fatter.

    Light from a near object needs more bending. The lens must be fatter. The contracted ciliary muscle lets the ligaments go slack, and the elastic lens becomes fatter.

    • If you chose B: When the ciliary muscle contracts, the ligaments slacken, not tighten. Then the lens springs fatter.
    • If you chose C: That is what happens when you look at something far away. A near object needs a fatter lens.

  5. Leila looks up at a tree on a far hill. What do the suspensory ligaments do?

    Tap your answer.

    1. Not this one. The ligaments slacken for near objects. For a far object they pull tight. Try another.

    2. Yes, that is right. For a far object the ciliary muscle relaxes, so the ligaments pull tight and stretch the lens thin. A thin lens bends light less, which suits light from far away.

    3. Not this one. The lens does not move. The ligaments change its shape. Try another.

    See the answer

    They pull tight, and the lens gets thinner.

    For a far object the ciliary muscle relaxes, so the ligaments pull tight and stretch the lens thin. A thin lens bends light less, which suits light from far away.

    • If you chose A: The ligaments slacken for near objects. For a far object they pull tight.
    • If you chose C: The lens does not move. The ligaments change its shape.

  6. Ravi can read a book clearly. The writing on the board at the front of the class is blurred. What is this called?

    Tap your answer.

    1. Not this one. Long sight is the other way round. Near things are blurred and far things are clear. Try another.

    2. Not this one. A damaged nerve would affect the whole picture, not only far things. Blurred far sight with a clear near sight is short sight. Try another.

    3. Yes, that is right. In short sight, light from far things meets in front of the retina, so far things look blurred. Near things are clear. Glasses with the right lens can fix this.

    See the answer

    Short sight

    In short sight, light from far things meets in front of the retina, so far things look blurred. Near things are clear. Glasses with the right lens can fix this.

    • If you chose A: Long sight is the other way round. Near things are blurred and far things are clear.
    • If you chose B: A damaged nerve would affect the whole picture, not only far things. Blurred far sight with a clear near sight is short sight.

  7. From an earlier chapter

    The pupil gets smaller in bright light. This is a reflex. Which part is the effector?

    Tap your answer.

    1. Not this one. The retina holds the receptors. They sense the light and start the impulse. Go to S4.5.2. Try another.

    2. Not this one. The optic nerve carries the impulses from the retina to the brain. It is a sensory path. Try another.

    3. Yes, that is right. An effector is a muscle or a gland that makes the response. The muscles of the iris change the size of the pupil.

    See the answer

    The muscles of the iris

    An effector is a muscle or a gland that makes the response. The muscles of the iris change the size of the pupil.

    • If you chose A: The retina holds the receptors. They sense the light and start the impulse. Go to S4.5.2.
    • If you chose B: The optic nerve carries the impulses from the retina to the brain. It is a sensory path.

  8. A pupil is 4 mm wide. What is its area in mm², to 1 decimal place? Use A = π × r × r.

    See the answer

    12.6 mm²

    The radius is 2 mm. A = π × 2 × 2 = 12.6 mm².

  9. A pupil widens from 2 mm to 6 mm. How many times bigger is its area?

    See the answer

    9 times

    The width is 3 times as big, and area goes with the width squared: 3 × 3 = 9. Check: π × 3 × 3 = 28.3 and π × 1 × 1 = 3.14, and 28.3 ÷ 3.14 = 9.

Keep it

Copy this into your exercise book.

Light goes in through the cornea and pupil, and the lens focuses it on the retina. Bright light: pupil small. Dim light: pupil wide. Near: ciliary muscle contracts, lens fatter. Far: muscle relaxes, lens thinner.

Draw: The eye cut across, with the cornea, pupil, lens, retina and optic nerve named.

Reading with a child? A short page for the adult: what this chapter teaches and three questions to ask.

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