EYE

• Eye


  Receives light and focuses it onto the retina at the back of the eye
• Retina


  Where the rod and cone cells are located
• Pupil reflex


  1. Protects the retina from damage in bright light

  2. Protects us from not seeing objects in dim light
• Pupil reflex


  • It's a reflex action so it happens automatically
  • Controlled by two groups of muscle; the radial muscle and the circular muscles
• In dim light


  The pupil dilates (widens) to allow as much light into the eye as possible
• In bright light


  The pupil constricts (narrows) to prevent too much light entering the eye and damaging the retina
• Accommodation


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

  2. The lens is elastic and its shape can be changed when the suspensory ligaments attached to it become tight or loose

  3. Changing the shape of the lens alters how much light is refracted

  4. The changes are brought about by the contraction or relaxation of the ciliary muscles
• To focus on a near object

  The ciliary muscles contract, the suspensory ligaments loosen, the lens is then thicker and refracts light rays more strongly
• Myopia


  Short-sightedness, rays of light do not focus on the retina
• Hyperopia


  Long-sightedness, rays of light do not focus on the retina
• Treating myopia and hyperopia

  1. Spectacle lenses (glasses) refract the light rays so they focus on the retina

  2. Hard and soft contact lenses sit on the surface of the eye and refract light

  3. Laser surgery changes the shape of the cornea to change the refractive power

  4. Lens replacement surgery replaces the lens with a plastic artificial lens
• Laser surgery for myopia slims down the cornea, reducing the refractive power
• Laser surgery for hyperopia changes the shape of the cornea to increase the refractive power
• Lens replacement surgery is more invasive than laser surgery and carries a risk of damage to the retina leading to complete sight loss