HUMAN EYE

• Eye


  Highly specialised sense organ containing receptor cells that allow us to detect the stimulus of light
• Cornea


  Transparent layer at front of eye which allows light through and refracts light
• Iris


  • Controls how much light enters the pupil
  • In bright light, circular muscles contract and radial muscles relax to make pupil smaller
  • In dim light, circular muscles relax and radial muscles contract to make pupil larger
• Lens


  Transparent curved surface in front of eye that changes shape to focus light onto retina for objects of different distances
• Pupil


  Hole that allows light to enter eye
• Optic nerve


  Sensory neurone that transmits visual information as electrical impulses between eye and brain
• Ciliary muscle


  • Ring of muscle that contracts and relaxes to change shape of lens
  • Allows us to focus on objects nearer or further away
• Suspensory ligaments


  • Connect ciliary muscles to lens
  • Along with ciliary muscle hold the lens in place
  • Allow us to focus on objects nearer or further away
• Blind spot


  Point at which optic nerve leaves eye where there are no receptor cells
• Sclera

  Protective outer layer of eyeball that helps keep eye in shape
• Fovea


  Region of the retina with the highest density of cones (colour detecting cells) where the eye sees particularly good detail
• Aqueous humour


  Watery liquid between cornea and lens
• Vitreous humour


  Jelly like liquid filling eyeball
• Refraction


  Occurs in cornea, lens and aqueous humour
• Accommodation


  1. Changing the shape of the lens to focus on near or distant objects

  2. Lens is elastic and its shape can be changed when the suspensory ligaments attached become tight or loose

  3. To focus on near object, ciliary muscles contract and suspensory ligaments loosen, lens becomes thicker and more curved

  4. To focus on distant object, suspensory ligaments tighten and the ciliary muscles relax, lens becomes taller and thinner
• Focal point


  The point where light rays cross over and focus in the eye
• Focal length


  The distance between the lens and the focal point
• Short-sightedness (myopia)


  • Eyeball is too long or lens is more curved than normal
  • Light is refracted too much
  • Focal point lies in front of the retina
• Focal point in front of retina


  Distant objects appear blurry
• Treatment for myopia


  Use contact lenses or glasses with concave lens
• Long-sightedness (hyperopia)


  • Lens is less curved than normal or eyeball is too short
  • Light is not refracted enough
  • Focal point falls behind the retina
• Focal point behind retina

  Close objects appear blurry
• Treatment for hyperopia


  Use contact lenses or glasses with convex lens
• Contact lenses


  Float on the surface of the cornea and work like glasses by focusing and refracting light
• Colour blindness


  • Cones in the retina do not work properly or are absent
  • Genetically inherited condition or can develop over time
  • Similar colours look alike
• There is no cure for colour blindness as the cone cells cannot be replaced
• Treatment for colour blindness


  Sufferers learn to live with the condition
• Cataracts

  • Build up of protein causes clouding of the lens
  • Cloudy lens means light is dispersed or absorbed rather than sharply focused
  • Often leads to blurred vision
  • Can lead to blindness if left untreated
  • Occurs over time or just with aging
• Treatment for cataracts


  Replace the cloudy lens with an artificial one
• Colour blindness DEFINITION


  Colour blindness is when a person cannot distinguish between certain colours and rarely cannot see colours at all
• concave and convex

  Concave is inwards and convex is outwards
• lens power formula


  lens power(dioptre D)=1/focal length(m)
• what type of power is concave

  negative lens power
• what type of power is convex

  positive lens power