Human eye

• • ebe


  A sense organ containing many receptors that are sensitive to light intensity / colour
• Light sensitive cells


  • Arranged together inside the eye in light sensitive layer (retina)
• Parts of the eye

  • Sensors
  • Pupil
  • Cornea
  • Lens
  • Sclera
  • Retina
  • Ciliary muscle
  • Blind spot
  • Optic nerve
• Sclera


  White outer layer of eye, tough and strong so not easy to damage. Transparent area at front called Cornea, which lets light into eye
• Cornea


  Changes direction of light rays, making sure focused on retina
• Iris


  Controls size of pupil, made up of contracting/relaxing muscles to change size of pupil and control amount of light reaching retina
• Lens


  Light passes through, held in place by suspensory ligaments and ciliary muscles. Lens fine tunes focusing of light rays, changing direction to produce clear image on retina
• Retina


  When light hits it, the light sensitive cells are stimulated which sends impulses to brain so you can see
• Where optic nerve leaves, eye has no retina so there is a blind spot
• Pupil size change in bright vs dim light

  1. Circular muscles contract, radial muscles relax, pupil constricts in bright light

  2. Circular muscles relax, radial muscles contract, pupil dilates in dim light
• Light entering eye


  Needs to be focused on light sensitive cells of retina so can see clearly. If light focused behind or in front, image will be blurred
• Focusing light on retina

  1. Light changes direction as it passes through cornea and lens, process of refraction

  2. Shape of lens can be changed by contraction/relaxation of ciliary muscles, which changes amount of lens refracts light
• Accommodation


  1. Distant - ciliary muscles relax, suspensory ligaments pull lens flat and thin

  2. Near - ciliary muscles contract, suspensory ligaments loosen, lens becomes thicker and more curved
• Short-sightedness (myopia)


  Lens too curved or eyeball too long, so light focused in front of retina. Corrected by concave glasses that spread light
• Long-sightedness (hyperopia)


  Lens too flat or eyeball too short, so light not focused on retina. Corrected by convex glasses that bend light
• Contact lenses and laser eye surgery can also correct vision problems
• Lens hardens with age, making accommodation more difficult