The Eye

• The eye is a sense organ
• The sclera is the tough, supporting wall of the eye

  
• The cornea is the transparent outer layer found at the front of the eye. It refracts light into the eye

  
• The iris contains muscles that allow it to control the diameter of the pupil and therefore how much light enters the eye

  
• The lens focuses the light onto the retina (which contains receptor cells sensitive to light intensity and colour)

  
• The shape of the lens is contorlled by the ciliary muscles and suspensory ligaments

  
• The optic nerve carried impulses from the receptors on the retina to the brain

  
• Very bright light can damage the retina - so you have a reflex to protect it

  
• When light receptors in the eye detect very bright light, a reflex is triggered that makes the pupil smaller. The circular muscles in the iris contract and the radial muscle relax. This reduces the amount of light that can enter the eye

  
• In dim light, the radial muscles contract and the circular muscles relax, which makes the pupil wider

  
• The eye focuses light on the retina by changing the shape of the lens - this is known as accommodation
• To look at near objects:

  • The ciliary muscles contract, which slackens the suspensory ligaments
  • The lens becomes flat (more curved)
  • This increases the amount by which it refracts light

  
• To look at distant objects:

  • The ciliary muscles relax, which allows the suspensory ligaments to pull tight
  • This makes the lens go thin (less curved)
  • So it refracts light by a smaller amount

  
• If the lens cannot refract the light by the right amount (so that it focuses on the retina), the person will be short- or long-sighted
• As you get older, your eye's lens loses flexibility, so it can't easily spring back to a round shape. This means light can't be focused well for near viewing, so older people often have to use reading glasses