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3.6.5Stoichiometry

Vision — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions

5 min read 4 parts Lesson 16 of 28 Unit quiz

📚 Cambridge IGCSE Chemistry 0620 Hub / Topical Revision
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Part 1 of 4Range of Vision

3.6.5 Vision

The range of vision is the set of distances over which the eye can produce a sharp image.

Near point
  • The closest distance at which a normal eye can focus clearly, which is about 25 cm in front of the eye.
Far point
  • The furthest distance at which a normal eye can focus clearly, which is infinity for a normal eye.
Range of vision
  • The distance between the near point and the far point, which for a normal eye runs from 25 cm to infinity.
Accommodation
  • The ability of the eye to change the thickness of its lens so that objects at different distances are focused on the retina.

Part 2 of 4Focusing on Near and Far Objects

1
To focus on a nearby object, the ciliary muscles contract, which makes the lens thicker and more powerful.
2
The thicker lens refracts the diverging rays from a near object more strongly, so they converge sharply onto the retina.
3
To focus on a distant object, the ciliary muscles relax, which makes the lens thinner and less powerful.
4
Rays from a distant object are almost parallel, so they need less refraction by the thinner lens to focus on the retina.

Part 3 of 4Defects of Vision

Defects of Vision
Long-sight
  • Light is focused behind the retina, so near objects look blurred because the near point is further than 25 cm.
Short-sight
  • Light is focused in front of the retina, so distant objects look blurred because the far point is less than infinity.
Cause of long-sight
  • The lens or cornea is too weak, or the eyeball is too short, so light is not refracted enough.
Cause of short-sight
  • The lens or cornea is too strong, or the eyeball is too long, so light is refracted too much.

Part 4 of 4Exam Checklist & Tips

3.6.5 – Key Points for the Exam

  • Near point The closest distance at which an eye can focus clearly.
  • Far point The furthest distance at which an eye can focus clearly.
  • Range of vision The distance between the near point and the far point.
  • Accommodation Changing the lens thickness to focus on objects at different distances.
  • Long-sightedness A defect in which near objects are blurred because the image forms behind the retina.
  • Short-sightedness A defect in which distant objects are blurred because the image forms in front of the retina.

Common Mistakes

  • Describing the lens as becoming thinner to focus on near objects; for near objects it becomes thicker.
  • Saying a normal eye has a finite far point; the far point of a normal eye is infinity.
  • Mixing up which defect blurs near objects and which blurs distant objects.
  • Stating that the ciliary muscles change the size of the eyeball; they change the shape of the lens.

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