the eye

• what is the sclera?

  the tough supporting wall of the eye
• what is the cornea?

  the transparent outer layer found at the front of the eye - it refracts light into the eye
• what is the iris?

  contains muscles that allow it to control the diameter of the pupil and therefore how much light enters the eye.
• what is the lens?

  focuses the light onto the retina which contains receptor cells sensitive to light intensity and colour
• the shape of the lens is controlled by ciliary muscles and suspensory ligaments
• the optic nerve carries impulses from the receptors to the brain
• how does the eye adjust to bright light?

  • when light receptors detect a bright light, a reflection is triggered that makes the pupils smaller. the circular muscles in the iris contract and the radial muscles relax. this reduces the amount of light that can enter the eye.
• how does the eye adjust to dim light?

  • the radial muscles contract and the circular muscles relax which makes the pupils wider so that more light can enter the eye
• how does the eye focus on near objects?

  • the ciliary muscles contract which slackens the suspensory ligaments
  • the lens becomes fat (more curved)
  • this increases the amount by which it refracts light
• how does the eye look at distant objects?

  • the ciliary muscles relax which allows the suspensory ligaments to pull tight
  • this makes the lens go thin (less curved)
  • so it refracts light by a smaller amount
• what is hyperopia?

  • occurs when the lens is the wrong shape and doesn't refract light enough or that the eyeball is too short
  • the image of near objects are brought into focus behind the retina
  • glasses with convex lenses are used to correct this
  • the lens refracts the light so light rays focus on the retina.
• what is myopia?

  • occurs when the lens is the wrong shape and refracts light too much or the eyeball is too long.
  • images of distant objects are brought to focus infant of the retina
  • glasses with a concave lens are used to correct this so that light rays focus on the retina
• how are contact lenses used to treat vision defects?

  • they sit on the surface of the eye and are shaped too combat the fault in focusing
  • they are popular because they are lightweight and invisible, they are more convenient than glasses for sport
  • soft lenses are more comfortable but carry a higher risk of eye infection than hard lenses
• how is laser eye surgery used to treat vision defects?

  • the laser vaporises tissue, changing the shape of the cornea and changing how strongly light refracts into the eye
  • slimming the cornea makes it less powerful to improve short sight. changing the shape so the its more powerful will improve long sight
  • complications = infection, negative eye reaction, making vision worse than before
• how does replacement lens surgery treat vision defects?

  • the natural lens of the eye is removed and an artificial lens made of clear plastic is inserted in its place.
  • this carries higher risks than laser eye surgery including damage to the retina which could lead to loss of sight