How the eye focuses: Homeostasis and response: Biology: GCSE (9:1)

• Pupil


  Centre of the iris, it varies in size to regulate the amount of light that reaches the retina

  
• Iris


  Made up of muscles called circular and radial muscles that work antagonistically to regulate the size of the pupil

  
• Reflex response to dim light

  Circular muscles relax, radial muscles contract, pupil dilates so that more light can enter the eye

  
• Reflex response to bright light

  Circular muscles contract, radial muscles relax, pupil constricts so that less light can enter the eye

  
• Cornea


  Focuses the entry of light into the eye but is fixed and unable to adjust its focus

  
• Lens


  Focuses light rays to the back of the eye and can change shape to adjust focus for distant and near objects

  
• Retina

  The light-sensitive surface at the back of the eye containing receptor cells that detect light

  
• Accommodation


  The process by which the eye lens changes shape to focus on near or distant objects

  
• Ciliary muscles


  Works with the suspensory ligaments and can contract or relax to adjust the shape of the lens

  
• Suspensory ligaments

  Works with the ciliary muscles and can loosen or be pulled tight to adjust the shape of the lens

  
• Focusing on a near object

  Ciliary muscles contract, suspensory ligaments loosen, lens is thicker and refracts light rays strongly

  
• Focusing on a distant object

  Ciliary muscles relax, suspensory ligaments pull tight, lens is pulled thin and only slightly refracts light rays

  
• Hyperopia (long-sightedness)


  Where a person has difficulty focusing on nearby objects because the light rays focus behind the retina

  
• Causes of hyperopia

  When the lens is too weak or the eyeball is too short

  
• Myopia (short-sightedness)


  Where a person has difficulty focusing on distant objects because the light rays focus in front of the retina

  
• Causes of myopia

  When the lens is too strong or eyeball is too long

  
• Convex lenses


  Used for correcting hyperopia by moving an image forward and focusing it on the retina

  
• Concave lenses


  Used for correcting myopia by moving an image further back and focusing it on the retina

  
• Modern sight treatments

  Hard and soft contact lenses, laser surgery and lens replacement