Eye Defects and Their Correction

1. 1 State [2 marks]

   State what a person who is short-sighted can see clearly and what they cannot.

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   Model answer

   They can see near objects clearly but not distant objects.

   Mark scheme

   • Near objects are clear — 1 mark
   • Distant objects are blurred — 1 mark
2. 2 Name [2 marks]

   Name the type of lens used to correct (a) short sight and (b) long sight.

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   Model answer

   (a) A concave lens. (b) A convex lens.

   Mark scheme

   • Concave for short sight — 1 mark
   • Convex for long sight — 1 mark
3. 3 Describe [3 marks]

   Describe what happens to light in the eye of a person with long sight.

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   Model answer

   The eyeball is too short, or the lens too weak, so the light is not bent enough. It would be focused behind the retina. The image on the retina is therefore blurred.

   Mark scheme

   • Light not bent enough — 1 mark
   • Focused behind the retina — 1 mark
   • Image blurred — 1 mark
4. 4 Explain [3 marks]

   Explain how a cataract affects vision and how it can be treated.

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   Model answer

   A cataract is a cloudy lens, which stops light passing through properly, so vision is blurred or misty. It can be treated by an operation to replace the cloudy lens with a clear artificial lens.

   Mark scheme

   • The lens becomes cloudy — 1 mark
   • Light cannot pass properly so vision is blurred — 1 mark
   • Replaced with an artificial lens — 1 mark
5. 5 Explain [4 marks]

   Explain how a concave lens corrects short sight.

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   Model answer

   In a short-sighted eye the light from a distant object is focused in front of the retina. A concave lens spreads the light rays out slightly before they enter the eye. The lens of the eye then focuses them further back, so that they meet on the retina and the image is sharp.

   Mark scheme

   • Light is focused in front of the retina — 1 mark
   • A concave lens spreads the rays out — 1 mark
   • The eye's lens focuses them further back — 1 mark
   • On the retina, giving a sharp image — 1 mark
6. 6 Explain [6 marks]

   Explain why colour blindness is more common in men than in women, and describe the effects of colour blindness and how it is diagnosed.

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   Model answer

   Colour blindness is caused by missing or faulty cone cells, so a person cannot tell some colours apart, most often red and green. The genes for these cone cells are carried on the X chromosome. A man has only one X chromosome, so one faulty gene makes him colour blind, whereas a woman has two, so a working gene on the other X usually hides the fault. It can be diagnosed with Ishihara plates, which hide numbers in coloured dots. There is no cure, but tinted lenses can help some people.

   Mark scheme

   • Missing or faulty cone cells — 1 mark
   • Cannot tell some colours (red and green) apart — 1 mark
   • Gene is on the X chromosome — 1 mark
   • Men have one X, so one faulty gene has an effect — 1 mark
   • Diagnosed using coloured plates — 1 mark
   • No cure / tinted lenses help — 1 mark

Quick check

1. 1

   A person cannot see distant objects clearly but can see near objects. What is the defect?

   1. ALong-sightedness
   2. BShort-sightedness
   3. CCataract
   4. DColour blindness
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   B: Short-sightedness

   This is short-sightedness (myopia). Distant objects are blurred because light is focused in front of the retina.

2. 2

   Where is the light focused in a long-sighted eye?

   1. AIn front of the retina
   2. BOn the retina
   3. CBehind the retina
   4. DOn the cornea
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   C: Behind the retina

   In long sight the light would be focused behind the retina, so the image is blurred.

3. 3

   Which lens corrects short sight?

   1. AConcave
   2. BConvex
   3. CFlat
   4. DCloudy
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   A: Concave

   A concave lens spreads light rays out so the eye can focus them on the retina.

4. 4

   Which lens corrects long sight?

   1. AConcave
   2. BCloudy
   3. CFlat
   4. DConvex
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   D: Convex

   A convex lens brings light rays together, so the eye can focus them on the retina.

5. 5

   What is a cataract?

   1. AA damaged retina
   2. BA cloudy lens
   3. CA faulty cornea
   4. DA missing cone cell
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   B: A cloudy lens

   A cataract is a clouding of the lens, which makes vision misty or blurred.

6. 6

   How are cataracts treated?

   1. AWith concave glasses
   2. BWith eye drops
   3. CBy replacing the lens
   4. DWith a tinted lens
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   C: By replacing the lens

   The cloudy lens is removed and replaced by a clear artificial lens.

7. 7

   What causes red-green colour blindness?

   1. AMissing or faulty cone cells
   2. BA cloudy lens
   3. CAn eyeball that is too long
   4. DA damaged optic nerve
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   A: Missing or faulty cone cells

   One or more types of cone cell are missing or do not work properly.

8. 8

   Which of these can cause short sight?

   1. AAn eyeball that is too short
   2. BA lens that is too weak
   3. CRod cells that are missing
   4. DAn eyeball that is too long
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   D: An eyeball that is too long

   If the eyeball is too long, light focuses before it reaches the retina.

9. 9

   Many older people need reading glasses. Which is the most likely reason?

   1. ATheir eyeball becomes longer
   2. BTheir lens becomes stiffer and cannot thicken
   3. CTheir cones stop working
   4. DTheir pupils become larger
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   B: Their lens becomes stiffer and cannot thicken

   The lens becomes stiffer with age and cannot get thicker to focus on near objects, so reading glasses with convex lenses help.