The Human Eye: Function (Edexcel IGCSE Biology: Double Science)

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 fatter
  • Light is refracted more

Diagram showing the eye when an object is close up

• 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

Diagram showing the eye when an object is far away

Focusing on distant and near objects table

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

The pupil reflex

The pupil reflex in dim light

The pupil reflex in bright light

Pupil reflex table