The eye

• The eye

  Muscles in the iris regulate the amount of light en entering the eye and reaching the retina

  In low light intensities, the radial muscles in the iris contract and the pupils dilate

  In bright lights the circular muscles contract and the pupil contracts
• Cones and rods


  Retina contains cones and rods

  Cones are sensitive to colour whilst Rods are sensitive to low light intensity (allow us to see in dim light)

  The receptor cells in the retina convert light into electrical impulses
• Accommodation

  The ability of the lens to change the shape and focus on objects at different distances
• Focus on near object

  Ciliary muscles contract

  Suspensory ligaments loosen

  Lens in then thicker and refracts light rays strongly

  Focus on distant object is opposite
• Eye


  Diagram

  
• Ciliary muscles


  Pulls the lens for focusing

  Changes the shape of the lens to focus light rays on the retina
• Cornea

  Let’s light into the eye and begins focusing

  Transparent region of the sclera at front of the eye - refracts light as it bends when it enters the eye
• Iris

  Controls the amount of light entering eye

  Has sets of muscles that control the size of the pupil and regulate the light reaching the retina
• Lens


  Focuses light rays on the retina
• Optic nerve

  Carries impulses from retina to brain
• Pupil


  Hole in the centre of the iris that allows light to pass through
• Retina

  Light sensitive layer and sends signals to the optic nerve
• Myopia- short sightedness


  Light rays converge before retina

  Out of focus and blurry

  Glasses with concave lens that spread the light from distant objects before it reaches the light
• Hyperopia - Long sightedness 


  Light converge after retina

  Lens can’t refract rays strong enough

  Light is focused behind retina - out of focus

  Glasses with convex lens which bring light rays together more
• Treatments

  Generally treated with spectacle lenses which refract light rays so they focus on retina

  Hard and soft contact lenses, laser surgery to change cornea shape and replacement lenses