How are cataracts usually treated?

By replacing the cloudy lens with an artificial lens

The eye: colour vision, defects and correction

The eye focuses light onto the retina. When the focusing goes wrong, lenses or surgery can often put it right.

GCSE · 1BI0 · 2.15B, 2.16B, 2.17B (Biology only)
About 15 minutes

Rods, cones and focusing

The retina has two kinds of light receptor: rods work in dim light but only see in black and white; cones work in bright light and detect colour.
To focus on a near object, the ciliary muscles contract, the suspensory ligaments slacken and the lens becomes fatter (more curved).
To focus on a distant object, the ciliary muscles relax, the ligaments pull tight and the lens becomes thinner.

Common defects

Defect	What goes wrong	Correction
Short-sightedness (myopia)	Distant objects are blurred; light focuses in front of the retina	Concave (diverging) lenses, or laser surgery to reshape the cornea
Long-sightedness (hyperopia)	Near objects are blurred; light focuses behind the retina	Convex (converging) lenses, or laser surgery
Cataracts	The lens becomes cloudy, so vision is blurred and colours look faded	Surgery to replace the cloudy lens with an artificial lens
Colour blindness	Some cones do not work properly, often for red and green	No cure; special tinted lenses can help some people

Glasses, contacts and surgery

Glasses and contact lenses bend light before it enters the eye so the image lands on the retina.
Laser surgery changes the shape of the cornea so it refracts light by the right amount.
Colour blindness is usually inherited.

Worked example

A student can read a book easily but cannot read the board at the front of the class. Name the defect and how it can be corrected.

Near objects are clear but distant ones are blurred.
Light from distant objects focuses in front of the retina.
A concave (diverging) lens spreads the light out first.

Answer: Short-sightedness, corrected with concave (diverging) lenses or laser surgery.

Key idea

Rods see in dim light, cones see colour. Short sight: focus in front of the retina, use a concave lens. Long sight: focus behind the retina, use a convex lens. Cataracts: cloudy lens, replaced by surgery. Colour blindness: faulty cones.

Quick check

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Extra practice

2 more questions

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In long-sightedness, where does light from a near object focus?
1. On the retina
2. On the cornea
3. In front of the retina
4. Behind the retina
Show the answer
Answer: Behind the retina
The lens is not strong enough or the eyeball is short.
Why does a convex lens correct long-sightedness?
1. It blocks light
2. It diverges light
3. It colours light
4. It converges light so it focuses on the retina
Show the answer
Answer: It converges light so it focuses on the retina
Extra converging brings the focus forward.

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Sources and references

Checked by the GCSE-ready editorial team, 17 October 2026. Last updated 2 October 2026. Spotted a mistake? Tell us and we will fix it.

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