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Biology · The Nervous System

The Eye

The eye is the sense organ that detects light. This lesson is for separate Biology. It covers the main structures of the eye and what they do, how the pupil and the lens respond to light and distance, and the common defects of the eye and how they can be corrected.

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  • 8 key terms
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Learning Objectives

  1. 1Explain how the main structures of the eye are related to their functions.
  2. 2Explain how the iris controls the amount of light entering the eye.
  3. 3Explain how the eye focuses on near and distant objects.
  4. 4Describe colour blindness, short-sightedness and long-sightedness, and explain how some of these problems can be overcome.

Retrieval practice

  1. 1

    What does a receptor do?

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    It detects a stimulus and starts an impulse.

  2. 2

    What is a reflex action?

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    A rapid, automatic response to a stimulus that protects the body.

  3. 3

    What does a motor neurone carry?

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    Impulses from the central nervous system to an effector.

  4. 4

    What is refraction?

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    The bending of light as it passes from one transparent material into another.

Seeing the world

The eye works like a camera. Light from an object enters the eye and is bent so that it forms a sharp image on the back of the eye, where receptor cells change the light into nerve impulses. The brain then interprets the impulses as a picture. The eye has to adjust to bright and dim light and to near and distant objects, and it can develop defects that stop the image from being sharp.

The structure of the eye

Each part of the eye has a job in getting a clear image onto the retina.

  • Cornea

    The transparent front of the eye. It refracts, or bends, the light and does most of the focusing.

  • Iris

    A ring of muscle that controls how much light enters the eye by changing the size of the pupil.

  • Pupil

    The hole in the middle of the iris that lets light into the eye.

  • Lens

    A transparent, flexible structure that changes shape to focus light on the retina.

  • Retina

    The layer at the back of the eye that contains the light receptor cells.

  • Optic nerve

    Carries impulses from the retina to the brain.

  • Ciliary body

    A ring of muscle round the lens that changes the shape of the lens.

  • Suspensory ligaments

    Fibres that connect the ciliary body to the lens and hold it in place.

Rods and cones

The retina contains two kinds of light receptor cell.

  • Rod cells

    Rods are very sensitive to light, so they let you see in dim light, but they do not detect colour.

  • Cone cells

    Cones need brighter light, and there are three kinds, each sensitive to red, green or blue light, which lets you see in colour.

  • Impulses

    When light hits a receptor cell it starts an impulse in a sensory neurone, which travels along the optic nerve to the brain.

Controlling the amount of light

Too much light would damage the retina, and too little would make it hard to see. The iris adjusts the size of the pupil in a reflex action.

  • Bright light

    The circular muscles of the iris contract and the radial muscles relax. The pupil becomes smaller, so less light enters the eye.

  • Dim light

    The radial muscles contract and the circular muscles relax. The pupil becomes larger, so more light enters the eye.

  • A reflex

    The change is automatic, and protects the retina from damage.

Focusing on near and distant objects

To see an object clearly its light has to be brought to a focus on the retina. The lens changes shape to do this, which is called accommodation.

  • Near object

    The ciliary muscles contract, which slackens the suspensory ligaments. The lens becomes thicker and more curved, so it bends the light more.

  • Distant object

    The ciliary muscles relax, which pulls the suspensory ligaments tight. The lens becomes thinner and less curved, so it bends the light less.

  • The result

    In both cases the light is focused on the retina and the image is sharp.

Near and distant objects

Looking at a near object

  • Ciliary muscles contract
  • Suspensory ligaments slacken
  • Lens becomes thicker and more curved
  • Light is bent more

Looking at a distant object

  • Ciliary muscles relax
  • Suspensory ligaments are pulled tight
  • Lens becomes thinner and less curved
  • Light is bent less

Defects of the eye

Some people cannot focus light properly, or cannot detect all colours.

  • Short-sightedness

    The person can see near objects clearly but not distant ones. The lens is too strong or the eyeball is too long, so light from distant objects focuses in front of the retina.

  • Long-sightedness

    The person can see distant objects clearly but not near ones. The lens is too weak or the eyeball is too short, so light from near objects focuses behind the retina.

  • Colour blindness

    The person cannot tell some colours apart, usually red and green, because one or more types of cone cell do not work. Glasses cannot correct it.

Overcoming eye defects

Several treatments can correct a focusing problem.

  • Spectacles

    A concave lens corrects short sight. A convex lens corrects long sight.

  • Contact lenses

    Thin plastic lenses worn on the surface of the eye do the same job.

  • Laser eye surgery

    A laser reshapes the cornea so that it bends light more or less.

  • Replacement lens

    A damaged or cloudy lens can be removed and replaced with an artificial lens.

Case study

Walking into a dark room

When you walk from a bright place into a dark room you can see almost nothing at first. The radial muscles of the iris contract at once and the pupils widen, which lets more light in. The rod cells then slowly become more sensitive to light, over several minutes. After a while you can see shapes in the dim light, though not in colour, because the cones need brighter light to work.

Pupils Widen quickly in a reflex
Rods Become more sensitive over minutes

The big idea

The cornea and the lens focus light on the retina, the iris controls how much light enters, and the ciliary muscles change the shape of the lens to focus on near and distant objects.

The eye

The eye

  • Cornea and lens

    • refract light to focus it on the retina

  • Iris and pupil

    • control the amount of light

  • Retina

    • rod and cone cells
    • optic nerve to the brain

  • Accommodation

    • ciliary muscles and suspensory ligaments change the lens

  • Defects

    • short sight
    • long sight
    • colour blindness

Summary and exam focus

  • The cornea and the lens refract light onto the retina, which has rod and cone cells.
  • The iris controls the size of the pupil, which controls how much light enters.
  • For a near object the ciliary muscles contract, the ligaments slacken and the lens is thick.
  • For a distant object the ciliary muscles relax, the ligaments are taut and the lens is thin.
  • Short sight is corrected with a concave lens, and long sight with a convex lens.
  • Colour blindness is caused by faulty cone cells.

Exam focus

Explain how the eye focuses on a near object. (4 marks) (4 marks)

Write the sequence in order: ciliary muscles contract, suspensory ligaments slacken, the lens becomes thicker and more curved, and so light is bent more.

Key terms

The words this lesson expects you to use. Each one is linked from the first place it appears above.

Cornea
The transparent front of the eye that refracts light.
Iris
The ring of muscle that controls the size of the pupil.
Pupil
The hole in the iris through which light enters.
Lens
The flexible structure that focuses light on the retina.
Retina
The layer at the back of the eye with the light receptor cells.
Accommodation
The change in the shape of the lens to focus on near or distant objects.
Ciliary muscles
The muscles that change the shape of the lens.
Suspensory ligaments
The fibres that hold the lens and are slack or taut as the ciliary muscles change.

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