The Eye (Cambridge O Level Biology)

• The eye is a sense organ containing receptor cells that are sensitive to light (rod cells) and colour (cone cells)

The eye diagram

The eye is adapted to detect light stimuli

Function of the eye table

The blind spot

• At the point where the optic nerve joins the retina, there are no light-sensitive rod and cone cells on that part of the retina
• Light falling onto that part of the retina will not result in an image being detected
  • the brain 'fills in' from surrounding light so we don't see a black hole where no light has fallen
• This causes a blind spot, where we cannot detect an object in our peripheral vision even if it is there

The pupil reflex

• This is a reflex action carried out to protect the retina from damage in bright light and to increase our ability to see objects in dim light
  • In bright light the pupil constricts (narrows) in order to prevent too much light entering the eye and damaging the retina
  • In dim light the pupil dilates (widens) in order to allow as much light into the eye as possible

Pupil reflex diagram

The pupil reflex protects the retina in bright light and maximises light reaching the retina in low light

Pupil reflex muscles diagram

Pupil reflex table

Accommodation: the function of the eye in focusing on near and distant objects

• The way in which 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

Focusing on a near object diagram

The ciliary muscles contact and the suspensory ligaments loosen to allow the eye to focus on a near object

• 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 a distant object diagram

The ciliary muscles relax and the suspensory ligaments tighten to focus on objects that are far away

Focusing on distant and near objects table