The Eye

• Parts of the eye:

  • sclera
  • cornea
  • iris
  • pupil
  • lens
  • retina
  • ciliary muscles
  • suspensory ligaments
  • optic nerve
• Sclera

  the tough, supporting wall of the eye
• Cornea

  the transparent outer layer at the front of the eye, which refracts light into the eye
• Iris

  contains muscles that allow it to control the diameter of the pupil and therefore how much light is let into the eye
• Pupil

  the hole in the middle of the eye that light enters through
• Lens

  focuses light onto the retina
• Retina

  contains receptor cells sensitive to light intensity and colour
• The shape of the lens in controlled by the ciliary muscles and suspensory ligaments
• Optic nerves

  carries impulses from the receptors on the retina to the brain
• Very bright light can damage the retina, so we have a reflex to protect it, called the iris reflex
• Bright light:

  • the circular muscles contract
  • the radial muscles relax
  • the pupil gets smaller and reduces the amount of light that can enter the eye
• Dim light:

  • the radial muscles contract
  • the circular muscles relax
  • the pupil gets wider and increases the amount of light that can enter the eye
• Accommodation

  changing the shape of the lens to focus light on the retina
• Looking at near objects:

  • the ciliary muscles contract, which relaxes the suspensory ligaments
  • the lens becomes fat (more curved)
  • this increases the amount by which the lens refracts light
• Looking at far objects:

  • the ciliary muscles relax, which tightens the suspensory ligaments
  • the lens becomes thin (less curved)
  • this makes the lens refract light by a smaller amount
• If the lens cannot refract the light by the right amount, the person will be short-sighted or long-sighted