the eye

• the eye is a sense organ containing receptors sensitive to light intensity and colour
• cornea - transparent layer (lens) that refracts light by a set amount as it enters the eye
• pupil - gap in the iris that changes shape based on light intensity to determine how much light enters the eye
• iris - a muscle that contracts or relaxes to change the shape of the pupil, and so how much light enters the eye
• lens - a transparent structure behind the pupil that refract light by variable amounts and focuses light onto the retina
• retina - sensory tissue that contains millions of light receptors that absorb light and convert it into an electrical impulse
• optic nerve - relays electrical impulses from the retina to the brain to form an image
• ciliary muscle - contracts or relaxes to change the shape and curvature of the lens (contracts makes it more curved, and relax makes it more flat and stretched) which affects the refraction of the lens
• sensory ligament - ligament that stretches or slackens when the ciliary muscle relaxes or contracts to adjust the thickness and curvature of the lens
• accommodation is the process of changing the shape of the lens to focus on near or distant objects
• to focus on a near object:

  • the ciliary muscle contracts
  • the suspensory ligament loosens
  • the lens is thicker and more curved, and refracts light rays more strongly
• to focus on a distant object:

  • the ciliary muscle relaxes
  • the suspensory ligament is stretched and pulled tight
  • the lens is pulled thin, which means it only slightly refracts light rays
• two common defects in the eye are myopia (short sightedness) and hyperopia (long sightedness) in which rays of light do not focus on the retina
• in bright light:

  • the circular muscles of the iris contracts
  • the radial muscles relax
  • makes the pupil smaller
  • so less light can enter
• in dim light:

  • the circular muscles of the iris relax
  • the radial muscles contract
  • the size of the pupil increases
  • so more light rays can enter the eye
• myopia:

  • struggle to see objects far away from them as the image is brought to focus in front of the retina
  • caused by the eyeball being elongated, meaning the distance between the lens and retina is too great
  • or by the lens being too curved and thick which leads to more refraction than necessary
  • glasses with concave lens corrects the refraction of light rays so they focus on the retina
• hyperopia:

  • struggle to see objects near them as the image focuses behind the retina
  • caused by the eyeball being too short, meaning the distance between the retina and and lens is too short
  • or by a loss of elasticity in the lens meaning it cannot become thick enough to focus (often age-related)
  • glasses with convex lens refract light rays more strongly so that they focus on the retina
• generally myopia and hyperopia are treated with spectacle lenses which refract the light rays so that they do focus on the retina
• new technologies to treat myopia and hyperopia now include:

  • hard and soft contact lenses -> sit on the surface of the eye (cornea) and help refract light properly
  • laser surgery -> changes the shape of the cornea to change the refracting power
  • for myopia, the cornea is slimmed down to reduce its refracting power
  • for hyperopia, the cornea shape is changed to increase its refracting power
  • a replacement lens in the eye -> a faulty lens is replaced by an artificial plastic lens