The Eye

• 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 is allowed into the eye
• the LENS focuses the light onto the RETINA
• the RETINA contains receptor cells sensitive to light and colour
• the shape of the lens is controlled by the CILIARY MUSCLES and SUSPENSORY LIGAMENTS
• When light receptors in the eye detect a very bright light, a reflex is triggered that makes the pupil smaller. The circular muscles in the iris contract and the radial muscles 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
• To look at near objects:

  1. The ciliary muscles contract, which slackens the suspensory ligaments
  2. the lens becomes fat (more curved)
  3. this increases the amount by which it refracts light
• To look at distant objects:

  1. the ciliary muscles relax, which pulls the suspensory ligaments taut
  2. The lens becomes thin (less curved)
  3. It refracts light by a smaller amount
• accommodation is the process of changing the shape of the lens to focus on near or distant objects
• two common defects of he eye are myopia (short sightedness) and hyperopia (long sightedness)
• these defects are generally treated with glasses, which refract the light rays so that they do focus on the retina
• New technologies now include hard and soft contact lenses, laser eye surgery to change the shape of the cornea and a replacement lens in the eye