The Eye

• Eye

  A sense organ that contains receptors sensitive to both light intensity and color of light
• How light passes through the eye

  1. Light passes through the transparent cornea

  2. Light passes through the pupil in the center of the iris

  3. Light passes through the lens

  4. Light is focused onto the retina at the back of the eye
• Cornea


  • Starts the focusing of light rays
• Lens


  • Can change shape to focus light from distant or near objects (accommodation)
• Retina


  • Contains receptor cells that detect light intensity and color
  • Sends electrical impulses down the optic nerve to the brain
• Sclera


  • Tough outer structure that protects the eye
• Ciliary muscles and suspensory ligaments


  • Work with the lens to allow focusing on distant or near objects
• Iris


  • Colored part of the eye
  • Controls the size of the pupil to regulate the amount of light entering the eye
• Pupil size adjustment in response to light

  1. Dim light: Pupil dilates to allow more light to enter

  2. Bright light: Pupil constricts to reduce light entering
• Pupil size adjustment is a reflex action not involving the conscious part of the brain
• Accommodation


  The ability to change the shape of the lens to focus on near or distant objects
• Accommodation

  1. Ciliary muscle contracts

  2. Suspensory ligaments loosen

  3. Lens becomes thicker

  4. Lens refracts light more strongly
• Focusing on distant objects

  1. Ciliary muscle relaxes

  2. Suspensory ligaments are pulled tight

  3. Lens becomes thinner

  4. Lens refracts light less strongly
• Focusing on near objects

  1. Ciliary muscle contracts

  2. Suspensory ligaments loosen

  3. Lens becomes thicker

  4. Lens refracts light more strongly
• Hyperopia (long-sightedness)


  People with long sight have no problem focusing on distant objects but cannot focus on nearby objects
• Causes of hyperopia

  • Eyeball too short so light focused behind retina
  • Lens becomes less elastic so cannot become thick enough
• Treatment for hyperopia

  Glasses with convex lenses to partially focus light before it enters the eye
• Myopia (short-sightedness)


  People with short sight can focus on nearby objects but cannot focus on distant objects
• Causes of myopia

  • Eyeball too long so light focused in front of retina
  • Lens too thick
• Treatment for myopia

  • Glasses with concave lenses to partially unfocus light before it enters the eye
  • Contact lenses
  • Laser surgery to change cornea shape