2.16B Describe defects of the eye including cataracts, long- sightedness,…

The idea

The eye can develop several defects that affect vision. Cataracts occur when the lens becomes cloudy, so light is scattered and vision becomes blurred; they can be treated by replacing the lens. Long-sightedness (hyperopia) means a person sees distant objects clearly but near objects are blurred, because the lens does not refract light strongly enough or the eyeball is too short; a converging (convex) lens corrects it. Short-sightedness (myopia) means near objects are clear but distant objects are blurred, because the lens refracts too strongly or the eyeball is too long; a diverging (concave) lens corrects it. Colour blindness is an inability to distinguish certain colours, usually caused by faulty cone cells in the retina, and it cannot be corrected by lenses. Each defect has a distinct cause and correction.

What earns the marks

• Cataracts: the lens becomes cloudy, causing blurred vision; treated by replacing the lens.
• Long-sightedness: distant objects are seen clearly but near objects are blurred; corrected with a converging (convex) lens.
• Short-sightedness: near objects are seen clearly but distant objects are blurred; corrected with a diverging (concave) lens.
• Colour blindness: difficulty distinguishing certain colours, usually due to faulty cone cells in the retina, and not corrected by lenses.
• Each defect has a distinct cause linked to the lens, eyeball shape or retina.

Common mistakes and how to put them right

• Reversing long-sightedness and short-sightedness.

  Put it right: Long-sightedness blurs near objects, short-sightedness blurs distant objects.

• Matching the wrong lens to a defect.

  Put it right: Converging (convex) lenses correct long-sightedness and diverging (concave) lenses correct short-sightedness.

• Thinking colour blindness can be corrected with glasses.

  Put it right: It results from faulty cone cells and is not corrected by lenses.

• Describing cataracts as a retinal problem.

  Put it right: Cataracts involve a cloudy lens.

Exam tips

• Identify the key symptom in the question (near or distant blurring) before selecting an option.
• Link each defect to its cause: lens cloudiness for cataracts, eyeball shape or lens strength for long- and short-sightedness, cone cells for colour blindness.
• Check that the correction named matches the defect, using converging lenses for long-sightedness and diverging lenses for short-sightedness.
• Eliminate options that claim colour blindness is corrected by lenses.