The eye — Edexcel GCSE Biology
How the cornea, lens, iris, rods and cones work, and the causes and correction of cataracts, long-sightedness, short-sightedness and colour blindness.
Revision notes
3 short notes, in the order of the specification. Each one in short:
The eye is a sensory receptor organ that detects light. The cornea and lens refract light to focus it on the retina, which contains rod and cone cells. Rod cells work in dim light, and cone cells detect colour in bright light. The iris changes the size of the pupil to control how much light enters the eye.
Common eye defects are cataracts, long-sightedness, short-sightedness and colour blindness. In cataracts the lens becomes cloudy. In long-sightedness, light from near objects is focused behind the retina; in short-sightedness, light from distant objects is focused in front of it. Colour blindness happens when some types of cone cell are missing or not working properly.
Eye defects are corrected so that light is focused on the retina. Long-sightedness is corrected with a convex lens, which bends light inwards, and short-sightedness with a concave lens, which spreads light out. Laser eye surgery can change the shape of the cornea instead. Cataracts are treated by replacing the cloudy lens with an artificial lens.
Exam questions
5 exam-style questions (12 marks), each with its mark scheme.
Answer the questionsFlashcards
12 cards: flip them, mark what you knew, and practise the rest.
Practise the cardsMind map
The whole of cells and control on one page, so you can see where this subtopic fits.
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Quick check questions
What does the iris do?
Show answer
It controls how much light enters the eye.
What is the difference between rod and cone cells?
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Rods work in dim light and do not detect colour. Cones detect colour and work in bright light.
What causes cataracts?
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The lens becomes cloudy, so less light reaches the retina.
What type of lens corrects short-sightedness?
Show answer
A concave lens.
Why can some people not tell red and green apart?
Show answer
One or more types of cone cell are missing or not working properly.
Frequently asked questions
How does the eye focus light?
The eye focuses light by refracting (bending) it onto the retina. The cornea refracts light as it enters the eye, and the lens refracts it further to focus it on the retina. Rod and cone cells in the retina then produce impulses, which travel to the brain along the optic nerve.
Why can we not see colour in dim light?
We cannot see colour well in dim light because only rod cells work then, and rods give black and white vision. Cone cells detect colour, but they need brighter light. There are three types of cone cell, sensitive to red, green or blue light, and together they give colour vision.
What causes short-sightedness?
Short-sightedness is caused by the eyeball being too long, or by the lens and cornea refracting light too much. Light from distant objects is focused in front of the retina, so distant objects look blurred while near objects are clear. It is corrected with a concave lens or by laser eye surgery.
How is long-sightedness corrected?
Long-sightedness is corrected with a convex lens, in glasses or contact lenses, which bends light inwards before it enters the eye. This makes light from near objects focus on the retina instead of behind it. Laser eye surgery can also be used to change the shape of the cornea so light focuses on the retina.
Why does the pupil get smaller in bright light?
The pupil gets smaller in bright light because the muscles of the iris reduce its size, so less light enters the eye and the retina is protected from damage. In dim light the iris makes the pupil larger, so more light enters and you can see better. The iris is the coloured ring around the pupil.
Written and checked against the Edexcel GCSE Biology (1BI0) specification · Updated October 2026