Correcting eye defects

Cells and control · The eye · note 3 of 3

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Correcting eye defectsSpec 2.17B

In short

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.

How eye defects are corrected
Defect	Correction
Long-sightedness	A convex lens (glasses or contact lenses) bends light inwards before it enters the eye, so it focuses on the retina instead of behind it.
Short-sightedness	A concave lens (glasses or contact lenses) spreads the light out before it enters the eye, so it focuses on the retina instead of in front of it.
Long- or short-sightedness (other option)	Laser eye surgery changes the shape of the cornea, so that light is focused on the retina.
Cataracts	Surgery removes the cloudy lens and replaces it with an artificial lens.

Ray diagrams: in long-sightedness light focuses behind the retina, and a convex lens in front of the eye makes it focus on the retina; in short-sightedness light focuses in front of the retina, and a concave lens makes it focus on the retina. — A **convex lens** corrects long-sightedness; a **concave lens** corrects short-sightedness. Both make light focus on the retina.

Exam tip:

Always say where the light is focused before the correction and where it is focused after. The aim is to focus it on the retina.

Quick check

What does the iris do?
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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?
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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.

Written and checked against the Edexcel GCSE Biology (1BI0) specification · Updated October 2026

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.

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