eyes

• Eye structure

  • Cross section of the eye
  • Light hits the cornea
  • Iris controls pupil size
  • Lens refracts light
  • Retina has cone and rod cells
  • Fovea has only cone cells
  • Optic nerve transmits impulses to brain
• Iris reflex

  1. Pupil constricts in bright light

  2. Pupil dilates in low light

  3. Circular muscles contract to constrict pupil

  4. Radial muscles contract to dilate pupil
• Bright light conditions

  Pupil constricts to prevent retina damage
• Low light conditions

  Pupil dilates to allow more light in
• Cone cells are sensitive to color, rod cells allow black and white vision in low light
• Fovea is a region of the retina with only cone cells, used for clearest vision
• Optic nerve transmits impulses from receptor cells in the retina to the brain
• Accommodation


  1. Reflex that changes the refractive power of the lens

  2. Allows seeing both near and distant objects
• Glasses

  • Help when the accommodation process doesn't work properly
• Cornea

  Refracts or bends light
• Lens

  Refracts or bends light
• Ciliary muscles


  Control the shape of the lens
• Suspensory ligaments


  Control the shape of the lens
• Focusing light on the fovea

  1. Light from object is refracted by cornea

  2. Lens fine-tunes refraction so light converges on fovea
• Lens shape for near objects

  1. Lens is short and fat

  2. More curved

  3. Refracts light more strongly
• Lens shape for distant objects

  1. Lens is stretched out

  2. Less curved

  3. Refracts light less strongly
• Ciliary muscle contraction

  1. Moves inwards towards lens

  2. Suspensory ligaments slacken

  3. Lens returns to fatter shape
• Ciliary muscle relaxation

  1. Moves away from lens

  2. Suspensory ligaments become taut

  3. Lens stretches out
• Long-sightedness (hyperopia)

  • Lens can't refract enough
  • Light focused behind retina
  • Nearby objects appear blurry
• Short-sightedness (myopia)

  • Lens refracts too much
  • Light focused in front of retina
  • Distant objects appear blurry
• Convex lenses


  Provide extra refracting power for long-sightedness
• Concave lenses

  Counteract over-refraction for short-sightedness