Pearson Edexcel · GCSE · Biology
Course version: 1BI0 · Version from Sept 2016
2.17B Explain how cataracts, long-sightedness and short-sightedness can be…
Full specification wording
Topic 2 – Cells and control: 2.17B Explain how cataracts, long-sightedness and short-sightedness can be corrected
This criterion asks you to link each eye defect to the structure at fault and then to the correction that restores a focused image on the retina.
Checked against the Pearson Edexcel 1BI0 specification (opens in a new tab) on .
- Point 16 of 16
Study notes
From the checked teaching notes for this specification point.
The idea
This criterion asks you to link each eye defect to the structure at fault and then to the correction that restores a focused image on the retina. A cataract is a cloudy lens, so the correction is surgery that replaces the lens with an artificial one, restoring light transmission. In long-sightedness the eye cannot focus on near objects because the lens does not refract light strongly enough or the eyeball is too short; a converging (convex) lens in glasses adds the extra refraction needed. In short-sightedness the eye cannot focus on distant objects because refraction is too strong or the eyeball is too long; a diverging (concave) lens spreads light before it enters the eye. In each case the correction moves the focal point back onto the retina.
What earns the marks
- Identifies a cataract as clouding of the lens and states that surgery replaces the lens with an artificial lens.
- States that long-sightedness is corrected with a converging (convex) lens that adds refraction so near objects focus on the retina.
- States that short-sightedness is corrected with a diverging (concave) lens that spreads light so distant objects focus on the retina.
- Links each defect to the position of the focal point relative to the retina and explains how the chosen correction moves it onto the retina.
Common mistakes and how to put them right
Reverses the lens types.
Put it right: Keep converging (convex) lenses for long-sightedness and diverging (concave) lenses for short-sightedness.
Treats a cataract as a problem of the retina.
Put it right: Describe it as clouding of the lens corrected by lens replacement surgery.
Claims a lens can cure short-sightedness by adding refraction.
Put it right: Explain that a diverging lens reduces the overall refraction so distant images focus on the retina.
Exam tips
- Match each defect to one named structure and one named correction before choosing an option.
- Check the direction of the error: too little refraction or too short an eyeball gives long-sightedness; too much refraction or too long an eyeball gives short-sightedness.
- Eliminate options that name the wrong lens shape, since converging and diverging lenses are easily swapped.
Try the lesson
The start of the Topic 2 – Cells and control: 2.17B Explain how cataracts, long-sightedness and short-sightedness can be corrected lesson, made from checked teaching notes for this specification point.
Have a go first
A person has long-sightedness, so nearby objects look blurred. Which correction helps this person see nearby objects clearly?
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