Eyes

• Cornea


  Transparent layer at the front of the eye

  Refracts light into the eye
• Iris 


  The coloured part of the eye

  Contains muscles that control the amount of light entering the eye
• Pupil


  The hole in the middle of the iris that lets the light in

  Diameter is controlled by the iris
• Lens


  A transparent, biconvex structure in the eye that refracts the eye into the retina
• Retina


  The thin layer of tissue at the back of the eye that contains the receptor cells for light and colour
• Sclera


  Tough white supporting the wall of the eye
• Ciliary muscles


  Muscles that are connected to the lens by the suspensory ligaments

  Change the shape of the lens
• Suspensory ligaments 


  These connect the lens to the ciliary muscles
• Optic nerve


  Carries impulses from the retina to the brain
• The eye


  A sense organ containing receptors sensitive to light and colour
• Accommodation 


  The process of changing the shape of the lens to focus on near or distant objects
• To focus on distant objects:

  • the ciliary muscles relax
  • the suspensory ligaments are pulled tight
  • the lens is pulled thin and only slightly refracts light rays
• To focus on near objects:

  • the ciliary muscles contract
  • the suspensory ligaments relax
  • the lens is thicker and refracts light rays more strongly
• Myopia (short-sightedness)

  • the eyeball is too long
  • light rays from distant objects focus in the eyeball in front of the retina
  • can be corrected with concave lenses
• Hyperopia (long-sightedness)

  • the eyeball is too short
  • light rays from distant objects focus behind the retina
  • can be corrected by convex lenses
• Advantages of contact lenses:

  • almost invisible
  • good for people playing sports
  • lightweight
• Disadvantages of contact lenses:

  • can cause irritation or infection
  • not permanent solution
• Contact lenses:

  • are inserted directly into the eye - sit onto the cornea
  • can be hard or soft
• Laser surgery:

  • the outer layer of the cornea is peeled back and lasers are used to change shape
• Advantages of laser surgery:

  • permanent solutions to correct vision
• Disadvantages of laser surgery:

  • surgical procedure which can cause infection
• Lens replacement:

  • surgery where an artificial acrylic or silicone lens is implanted into the eye
• Advantages of lens replacement

  • permanent solution to correct vision
• Disadvantages of lens replacement

  • a surgical procedure so it can cause infection