The Human Eye: Function (Edexcel IGCSE Science)

Spec point — 3.42 · understand the function of the eye in focusing on near and distant objects, and in responding to changes in light intensity

The Human Eye: Function

The function of the eye in focusing on near and distant objects

• The way the lens brings about fine focusing is called accommodation

• The lens is elastic and its shape can be changed when the suspensory ligaments attached to it become tight or loose

• The changes are brought about by the contraction or relaxation of the ciliary muscles

• When an object is close up:

  • The ciliary muscles contract (the ring of muscle decreases in diameter)
  • This causes the suspensory ligaments to loosen
  • This stops the suspensory ligaments from pulling on the lens, which allows the lens to become thicker
  • Light is refracted more

• When an object is far away:

  • The ciliary muscles relax (the ring of muscle increases in diameter)
  • This causes the suspensory ligaments to tighten
  • The suspensory ligaments pull on the lens, causing it to become thinner
  • Light is refracted less

Focusing on distant and near objects table

Exam Hint A common mistake is to refer to the suspensory ligaments contracting and relaxing. This is incorrect because suspensory ligaments are not muscles. You must say that the ligaments tighten or loosen/slacken instead.

The function of the eye in responding to changes in light intensity

• The pupil reflex is a reflex action carried out to protect the retina from damage

  • In dim light, the pupil dilates (widens) in order to allow as much light into the eye as possible to improve vision
  • In bright light, the pupil constricts (narrows) in order to prevent too much light from entering the eye and damaging the retina