👁 Cornea
The clear, curved window at the front of the eye. It lets light in and bends (refracts) it so it starts to focus.
Co-ordination and Response » Structure and Function of the Eye
What you'll learn this session
Study time: 30 minutes
Edexcel spec: 2.91
- Why the eye is a receptor
- The name and job of each part of the eye
- The difference between rods and cones
- What the fovea, optic nerve and blind spot are
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Unlock This CourseThe eye as a receptor
In the lesson Stimulus, Response and Homeostasis you met receptors. The eye is a receptor organ. Its stimulus is light. The eye collects light, focuses it, and turns it into electrical impulses that travel to the brain. This lesson is about the parts that do this, and what each one does. In the exam you may be asked to label a diagram of the eye, so learn the names carefully.
Key terms:
- Retina: the layer at the back of the eye that contains the light receptor cells.
- Rods: receptor cells in the retina that work in dim light and do not detect colour.
- Cones: receptor cells in the retina that work in bright light and detect colour.
- Fovea: the small area of the retina with the most cones, where vision is sharpest.
- Optic nerve: the nerve that carries impulses from the retina to the brain.
- Blind spot: the place where the optic nerve leaves the eye. It has no receptor cells.
The front of the eye
Light meets these parts first, in this order: the cornea, then the pupil (the opening in the iris), then the lens.
🌈 Iris
The coloured ring of muscle. It controls how much light gets into the eye.
🕳 Pupil
The hole in the middle of the iris. Light passes through it. It looks black because the light goes inside the eye.
🔍 Lens
A clear, flexible structure behind the pupil. It bends light to focus it onto the retina.
💪 Ciliary muscles
A ring of muscle around the lens. When they contract or relax, the shape of the lens changes.
🔗 Suspensory ligaments
Thin fibres that join the lens to the ciliary muscles. They hold the lens in place.
How the ciliary muscles and ligaments change the lens, and how the iris changes the pupil, are covered in the next lesson, Focusing and Responding to Light.
The back of the eye
After the lens, light travels through the inside of the eye and lands on the retina.
🎬 Retina
The layer lining the back of the eye. It contains the receptor cells that detect light. When light hits them, they start impulses.
🎯 Fovea
A small spot on the retina packed with cones. Light focused here gives the clearest, most detailed image.
🧠 Optic nerve
Carries impulses from the retina to the brain. The brain turns them into the picture you see.
Rods and cones
The retina has two types of light receptor cell.
🌙 Rods
Work well in dim light, so they help you see at night. They do not detect colour, so everything looks grey.
🌈 Cones
Work in bright light. They detect colour and give a sharp image. There are the most cones in the fovea.
This is why a dark room looks colourless. There is not enough light for the cones, so you rely on the rods.
The blind spot
The optic nerve leaves the back of the eye at one place. At that place there are no rods or cones, so light that lands there cannot be detected. This is the blind spot. You do not normally notice it, because your two eyes cover each other's gaps and the brain fills in the picture.
Learn this: the path of light
cornea → pupil (in the iris) → lens → retina (rods and cones) → impulses along the optic nerve → brain
Common mistakes
- Saying the pupil is a structure. It is a hole. The iris is the muscle around it.
- Mixing up rods and cones. Remember: cones = colour and bright light. Rods = dim light.
- Saying the lens does all the focusing. The cornea bends light too, and the lens focuses it onto the retina.
- Writing that the optic nerve detects light. It only carries impulses. The receptor cells are in the retina.
- Spelling "ligament" or "ciliary" wrongly. Examiners may not give the mark.
Exam-style question
(a) Name the part of the eye that contains the light receptor cells. [1 mark]
(b) Describe the job of the iris. [1 mark]
(c) A person walks from a bright street into a dark cinema. Explain why they cannot see colours clearly at first. [2 marks]
Model answer
(a) The retina (1)
(b) It controls how much light enters the eye (1)
(c) Cones need bright light to work (1), and in dim light only the rods work, which do not detect colour (1)
Exam tip
For "describe the job" questions, name the part and say what it does to the light, for example "bends light" or "controls the amount of light". Just writing "helps you see" earns nothing.