The Eye

• Conjunctiva

  lubricates and protects the surface of the eye
• sclera


  the tough outer layer that protects the eye
• cornea


  refracts light into the eye

  it is transparent and has no blood vessels to supply it with oxygen so oxygen diffuses from the outer surface
• iris 


  controls the diameter of the pupil and therefore how much light enters the eye
• pupil 


  the hole in the middle that lets light into the eye
• lens


  focuses light onto the retina
• retina


  the light sensitive part

  it is covered in light receptors called rods and cones
• Rods vs Cones


  Rods are more sensitive in dim light but can't sense colour

  Cones are sensitive to colours but arent so good in dim light

  Cones are found all over the retina, but there are loads of them at the fovea
• optic nerve


  carries impulses from the receptors to the brain
• the iris reflex - adjusting for bright light


  very bright light can damage the retina so you have to protect it

  very bright light triggers a reflex that makes the pupil smaller, allowing less light in - the brain tells the circular muscles in the iris to contract making the pupil smaller
• the iris reflex - very dim light


  the opposite happens in dim light - this time the brain tells the radial muscles to contract, which makes the pupil bigger
• how does the eye focus light on the retina?


  by changing the shape of the lens - this is known as accommodation
• To look at distant objects

  • the cilliary muscle relaxes, which allows the suspensory ligaments to pull tight
  • this makes the lens go thin (less curved)
  • so it refracts light by a smaller amount
• To look at nearer objects

  • The cillary muscle contracts, which slackens the suspensory ligaments
  • the lens becomes flat (more curved)
  • this increases the amount by which it refracts light
• Why are short sighted people unable to focus on distant objects?

  • their cornea or lens refracts the light too much
  • or their eyeball is too long
  • therefor the images of distant objects are brought into focus in front of the retina
• Why are long sighted people unable to focus on near objects?

  • their cornea or lens doesn't refract the light enough
  • or their eyeball is too short
  • therefor the images of near object are brought into focus behind the retina.