👁 Cornea
The clear, curved window at the front of the eye. It lets light in and refracts (bends) it so it starts to focus. It must be transparent so that light can pass through.
The Human Nervous System » The Eye
What you'll learn this session
Study time: 30 minutes
AQA spec: 4.5.2.3 (biology only)
- The parts of the eye and how each one's structure suits its job
- How the eye adapts to dim light and focuses on near and distant objects
- What short sight and long sight are, and how ray diagrams show them
- How lenses and new technologies correct these eye defects
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Unlock This CourseThe eye as a sense organ
The eye is a sense organ. It contains receptors that are sensitive to light intensity (how bright the light is) and to colour. When light hits these receptors, they send electrical impulses along a sensory neurone to the brain, which makes sense of what you are seeing.
You need to be able to name the main structures of the eye on a diagram and explain how each structure's design suits its function.
Key terms:
- Sense organ: an organ that contains receptors for a particular type of stimulus.
- Refract: to bend light rays as they pass from one transparent material into another.
Structures of the eye
⚪ Iris
The coloured ring of muscle around the pupil. It controls how much light enters the eye by changing the size of the pupil (the hole in the middle).
🔮 Lens
A clear, flexible structure behind the iris. It refracts light so that it focuses on the retina. Its shape can change, which lets you focus on near or distant objects.
🌟 Retina
The light-sensitive layer at the back of the eye. It contains the receptor cells that detect light intensity and colour, and it is where the image is focused.
Optic nerve
A bundle of sensory neurones. It carries impulses from the retina to the brain.
Sclera
The tough, white outer layer. It protects the eye and holds its shape.
Ciliary muscles and suspensory ligaments
The ciliary muscles form a ring around the lens. The suspensory ligaments connect the ring of muscle to the lens. Together they change the shape of the lens.
Adapting to dim light
Too much light can damage the retina, but in dim light the eye needs to take in as much light as it can. The iris adjusts the size of the pupil to control this.
- Bright light: the pupil becomes smaller, so less light enters the eye and the retina is protected.
- Dim light: the pupil becomes larger (wider), so more light enters the eye and you can see better.
This is a reflex, so it happens automatically. You have probably noticed your eyes taking a moment to adjust when you walk into a dark room.
Accommodation: focusing near and far
Accommodation is the process of changing the shape of the lens to focus on near or distant objects. The ciliary muscles and suspensory ligaments do this job.
📖 Focusing on a near object
The ciliary muscles contract. The suspensory ligaments loosen. The lens becomes thicker and refracts light rays strongly.
🏔 Focusing on a distant object
The ciliary muscles relax. The suspensory ligaments are pulled tight. The lens is pulled thin and only slightly refracts light rays.
Close objects send out light rays that spread out a lot, so the lens has to bend them strongly. Distant objects send light rays that are almost parallel, so the lens only needs to bend them a little.
Common mistakes
Students often mix up the muscles and the ligaments. For a near object, the muscles contract, which makes the ligaments loosen. The ligaments do not do the contracting. Also, a thick lens is for near objects, and a thin lens is for distant objects.
Eye defects: short sight and long sight
Two common defects of the eye are myopia (short sightedness) and hyperopia (long sightedness). In both, light rays do not focus on the retina, so the image is blurred.
- Myopia: a person can see near objects clearly but distant objects are blurred. The light rays from a distant object are focused in front of the retina.
- Hyperopia: a person can see distant objects clearly but near objects are blurred. The light rays from a near object are focused behind the retina.
Ray diagrams and spectacle lenses
You may be asked to interpret a ray diagram. Look at where the rays meet.
- Myopia: the rays cross before they reach the retina. A concave lens (thinner in the middle) spreads the rays out a little before they enter the eye, so that they then focus on the retina.
- Hyperopia: the rays have not yet met when they hit the retina. A convex lens (thicker in the middle) bends the rays inwards before they enter the eye, so that they focus on the retina.
Generally these defects are treated with spectacle lenses, which refract the light rays so that they do focus on the retina.
Worked example
A ray diagram shows light from a distant object meeting at a point in front of the retina, then spreading out again.
Step 1: The rays focus in front of the retina, so the defect is myopia.
Step 2: The rays need to be spread out slightly before they enter the eye, so the correcting lens is a concave lens.
Step 3: With the lens, the rays focus on the retina and the image is sharp.
New technologies
Newer technologies can also treat these defects:
- Contact lenses: hard and soft lenses worn on the surface of the eye, which refract the light rays so they focus on the retina.
- Laser surgery: a laser changes the shape of the cornea so that light is refracted correctly.
- Replacement lens: the lens inside the eye is replaced with an artificial lens.
Exam-style question
(a) Name the part of the eye that contains the receptor cells. [1 mark]
(b) A person walks from bright sunlight into a dim room. Describe what happens to the pupil and explain why. [2 marks]
(c) Explain how the lens changes shape when a person looks at a near object. [3 marks]
Model answer
(a) The retina (1)
(b) The pupil becomes larger (1) so that more light can enter the eye (1)
(c) The ciliary muscles contract (1), the suspensory ligaments loosen (1), so the lens becomes thicker and refracts light rays more strongly (1)
Exam tip
In part (c), give the ciliary muscles first, then the ligaments, then the lens. Writing the steps in order makes each mark point easy for the examiner to find.