the eye

• the eye is a sense organ containing receptors sensitive to light intensity and colour (of light)
• cornea:

  light rays pass through the transparent front of the eye

  it starts the focusing of the light rays
• pupil:

  light rays pass through

  this is in the centre of the iris (coloured part of the eye)
• lens:

  focus the light rays onto the back of the eye

  it can change its shape, allowing it to focus on distant or near objects Aka accommodation
• retina:

  where the light rays are focus too on the back of the eye

  it contains receptor cells for light, allowing it to detect light intensity and light colour
• optical nerves:

  the receptor cells in the retina send electrical impulses down the optic nerve to the brain
• process of how the eye detects light:

  • cornea
  • pupil
  • lens
  • retina
  • optical nerve
• sclera

  white part of the eye

  tough outer structure protects the eye
• ciliary muscles and suspensory ligaments

  work together with the lens

  to allow focus on distant and near objects
• iris is coloured part of the eye which controls the size of the pupil

  pupil is in the centre of the iris where light passes through
• adaptation to dim light:

  dark room = small amount of light entering the eye

  drop in light intensity is sensed by light receptors in the retina

  these send electrical impulses to the brain

  the brain sends electrical impulses to specific muscles in the iris

  these muscles contract , pupil = larger = more light entering the eye
• the adaptation to dim light is a reflex action as it does not involve the conscious part of the brain
• if walking into a bright room the reflex causes the pupil to become smaller, reducing the amount of light entering the eye thus protecting it from damage
• ACCOMMODATION is the process of changing the shape of the lens to focus on near or distant objects
• the lens is surrounded by a circular muscle called the ciliary muscle

  this is connected by fibres called suspensory ligaments

  by contracting/relaxing the ciliary muscles can change the thickness of the lens
• to focus on near object:

  • ciliary muscles contract
  • suspensory ligaments loosen
  • the lens is thicker
  • and refracts light rays more strongly
  • = focused to a point on the retina
• to focus on a distant object:

  • the ciliary muscles relax
  • the suspensory ligaments are pulled tight
  • the lens is pulled thin
  • and only slightly refracts light rays
  • = focused to a point on the retina
• two common defects in the eye:

  • myopia (short-sightedness)
  • hyperopia (long-sightedness)

  in which rays of light do not focus on the retina
• people with hyperopia cannot focus on nearby objects

  this is because:

  • the eyeball is too short = light is focused at a point BEHIND the retina
  • in elderly, lens becomes less elastic = cannot become thick enough to focus on near objects
• hyperopia can be treated with:

  • glasses with CONVEX LENSES = partially focus the light before it enters the eye

  
• people with myopia cannot focus on distant objects

  this is because:

  • the eyeball is too long = light is focused at a point INFRONT of the retina
  • lens is too thick = light is focused INFRONT of the retina
• myopia can be treated with:

  • glasses with CONCAVE lenses = partially UNFOCUS the light before it enters the eye so light rays focus on the retina

  
• this diagram shows: myopia because the light is focused INFRONT of the retina

  
• this diagram shows: hyperopia because the lights focused BEHIND the retina

  
• new technologies now include hard and soft contact lenses (instead of glasses)

  they refract the light
• new treatments for long and short-sightedness include laser surgery

  which changes the shape of the cornea, so it refracts the light to a greater or lesser extent
• new technology means that sometimes the lenses inside the eye can be replaced by using an artificial lens