The Eye

• The eye is a sense organ containing receptor cells that are sensitive to light and colour.
• Parts of eye are - suspensory ligament, cornea, iris, pupil, lens, ciliary muscle, scelera, retina, fovea and optic nerve.
• Cornea is the transparent layer of the eye that refracts light to focus it on the retina.
• Iris controls how much light enters the eye.
• Lens is a transparent disc that can change shape to focus light onto retina.
• Retina contains light receptor cells (photoreceptors) which are -

  Rods - detect light intensity

  Cones - detect colour
• Optic nerve is the sensory neuron that carries impulses between the eye and the brain.
• Pupil is the hole that allows light to enter the eye.
• The blind spot is a point where optic nerve joins retina and there are no rod or cone cells there.
• We cannot detect an object in our peripheral vision due to the blind spot.
• The pupil reflex is a reflex action carried out to protect the retina and allow us to see in dim light.
• In dim light, the pupil dilates to allow as much light as possible into the eye
• In bright light, the pupil constricts to prevent too much light entering and damaging the retina.
• Pupil reflex is also when a pair of antagonistic muscle groups act together to regulate the amount of light entering the eye.
• The muscles involved in the pupil reflex are the radial muscles and the circular muscles.
• In dark environment,

  1. Photoreceptors detect change
  2. Radial muscles contract
  3. Circular muscles relax
  4. Pupil dilates
  5. More light enters eye
• In bright environment,

  1. Photoreceptors detect change
  2. Radial muscles relax
  3. Circular muscles contract
  4. Pupil constricts
  5. Less light enters eye
• Accommodation is the function of the eye focusing on near and far objects.
• The lens is elastic and its shape can be changed when the suspensory ligaments attached to it becomes tight or loose.
• Change in shape of lens is brought about by the contraction or relaxation of the ciliary muscles.
• When an object is close up,

  1. Ciliary muscle contracts (ring of muscle decrease in diameter)
  2. Causes suspensory ligament to loosen
  3. Leads to rounder lens
  4. Light is refracted more
• When an objects is far away,

  1. Ciliary muscle relax (ring of muscle increase in diameter)
  2. Causes suspensory ligaments to tighten
  3. Leads to thinner lens
  4. Light is refracted less
• Rod cells are sensitive to light.
• Cone cells can distinguish different colours in bright light.
• The three types of cone cells are each sensitive to a different colour - red, blue, green.
• The fovea is an area on the retina where almost all cone cells are found.
• Rod cells are found all over retina (except in the blind spot)
• Eye Diagram

  A) Suspensory Ligament

  B) Cornea

  C) Iris

  D) Pupil

  E) Lens

  F) Ciliary Muscle

  G) Scelera

  H) Retina

  I) Fovea

  J) Optic Nerve