Defects of the eye

Cells and control · The eye · note 2 of 3

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Defects of the eyeSpec 2.16B

In short

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.

Defects of the eye
Defect	What happens
Cataracts	The lens becomes cloudy, so less light reaches the retina. Vision is blurred and can be lost.
Long-sightedness	The person can see distant objects clearly but not near objects. Light from near objects is focused behind the retina.
Short-sightedness	The person can see near objects clearly but not distant objects. Light from distant objects is focused in front of the retina.
Colour blindness	The person has difficulty telling some colours apart, such as red and green, because one or more types of cone cell are missing or not working properly.

Long-sightedness happens when the eyeball is too short, or the lens and cornea refract light too little. Short-sightedness happens when the eyeball is too long, or the lens and cornea refract light too much.

Common mistake:

Do not mix up the two. Long-sighted people have a problem with near objects. Short-sighted people have a problem with distant objects.

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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