the eye

• • the eye is a sense organ containing receptors sensitive to light intensity and colour
• • the sclera is the tough supporting wall of the eye
• • the cornea is the transparent outer layer at the front of the eye
  • it refracts light into the eye
• • the iris contains muscles that allow it to contorl the diameter of the pupil
• • the lens focuses light into the retina
  • the retina contains receptor cells sensitive to light intensity and colour
• • the ciliary muscles and suspensory ligaments control the shape of the lens
• • the optic nerve carries impulses from the receptors on the retina to the brain
• bright light:

  • light receptors in eye detect bright light
  • so, a reflex is trigerre to make the pupil smaller
  • the circular muscles in the iris contract
  • the radial muscles relax
  • this reduces the amount of light that enters the eye
• dim light:

  • light receptors in eye detect dim light
  • so, a reflex is triggered to make pupil wider
  • the circular muscles in the iris relax
  • the radial muscles contract
  • this increases the amount of light that can eneter the eye
• focus on a near object:

  • the ciliary muscles contract
  • the suspensory ligaments loosen
  • the lens becomes thicker and more curved
  • so, the lens refracts light rays strongly
• focus on a distant object:

  • the ciliary muscles relax
  • the suspensory ligaments are pulled tight
  • so, the lens becomes thin and less curved
  • so, the lens only slightly refracts light rays
• • as people age, the lens loses flexibility
  • so, it becomes harder for the lens to become curved
  • so, light cannot be focused well for near viewing
  • so, odler people often have to use reading glasses
• long sightedness:

  • long sightedness is known as hyperopia
  • long sighted people are unable to focus on near objects
  • the lens is in wrong shape, so, doesn't refract light enough into retina
  • images of near objects focus behind the retina
  • convex lenses are used to correct hyperopia
  • the lens refracts light rays so they focus on retina
• short sightedness:

  • short sightedness is known as myopia
  • unable to focus on far objects
  • the lens is in wrong shape, so refracts light too much
  • the images of dstant objects focus infront of retina
  • concave lenses correct myopia
  • the light rays made to focus on retina
• three treatments used to correct vision defects are:

  • contact lenses
  • laser eye surgery
  • replacement lens surgery
• • contact lens are thin lenses that sit on surface of eye
  • they are lightweight and invisible
  • they are more convienient than glasses for physical activities
  • soft contact lenses are more comfortable than hard contact lenses
  • there is high risk of eye infection with soft contact lens than hard contact lenses
• • laser eye surgery involves using a laser vaporise tissue, to change shape of cornea
  • this changes how strongly cornea refracts light into eye
  • slimming down cornea makes cornea less powerful and improves short sight
  • changing shape of cornea to make it more powerful will increase long light
  • there is risk of eye infection and making vision worse
• • replacement lens surgery can be used to treat long sightedness more effectivel
  • the natural lens of the eye is removed and artifical lens made of clear plastic is inserted
  • replacing lens has more risks than laser eye surgery
  • it involves work inside the eye
  • so, damage could be done to retina which could lead to loss of sight