THE EYE

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					                    The Eye
• Is an optical instrument that we used to see
  around us.
• It acts very much like a camera
  – It uses a converging lens to focus the image on a
    sensor
  – Lets more or less light in when needed
  – Allows us to view the image
Eye and Camera
                     Eye Anatomy




• Iris  opens or closes to let or more less light in
• Cornea and Lens  act to converge light rays to create an image
• Retina  screen where image is located. Creates an electrical
  signal to be interpreted by the brain.
• Optic nerve  transmits the electrical signal created by the
  retina to the brain so that we “see”
Blind Spot – Try It!
                     Blind Spot
• There are no light
  sensitive areas where the
  optic nerve connects to
  the retina
• If an image is formed on
  the blind spot we cannot
  see it
• The reason we normally
  do not notice our blind
  spots is because our other
  eye compensates and
  “fills in” the gap
                    Eye vs. Brain
• We don’t actually see with our eyes
• We use our eyes to create an image on our retina
• The eye uses a converging lens so the image produced on
  the retina is always smaller, inverted and real
• Computer Demonstration – Showing smaller
  inverted image with converging lens

• http://www.phy.ntnu.edu.tw/ntnujava/index.
  php?PHPSESSID=643ae5051e223f1f696d419c
  040b986d&topic=48.msg297#msg297
• These images are sent via electrical signals to
  the brain to be interpreted

• The brain then takes the inverted image and
  flips it so what we “see” is an upright image
          Camera Lens vs. Eye Lens
• Both cameras and eyes cannot move their “screen”
• Cameras move their lens to focus an image
• Eyes cannot move their lens
• Eyes instead change the shape of their lens using
  ciliary muscles which alters the focal length allowing
  focusing on the retina
• This is called Accommodation

                     Computer Simulation to show moving
                     screen, camera lens and Eye
Accommodation
                  Focusing
• Some people cannot focus incoming light rays as
  well as they should, creating blurred images

• This can happen when objects are far away or
near
     Myopia (Near-sightedness)
• The eye can focus light ray from nearby
  objects but cannot see objects at a distance
• Caused by a too-long eye ball or to much
  converging of light by the lens and cornea
• Light is focused in front of the retina
• A diverging lens is needed to spread apart the
  light rays so that they create an image at the
  retina
• The diverging ray is altered slightly and called
  a negative meniscus so that it looks better




         Computer Simulation to show Myopia
     Hyperopia (Far-sightedness)
• Person can see distant objects (a) but cannot see
  near objects (b)
• The eye cannot refract light well enough to create a
  focused image on the retina
• Image is created behind the retina  blurry
• Usually caused by the eye ball being too short




              Computer Demonstration to show Hyperopia
Converging Lens – Positive Meniscus
• Since light isn’t refracted well enough (a), far-
  sighted people can use a converging lens to
  help out (b)
(a)                          (b)
                Positive Meniscus
• The converging lens is altered to create a positive
  meniscus
• A positive meniscus is much more appealing shape
• It is still a converging lens because it is thicker in the
  middle then on the edges
                 Presbyopia
• A form of farsightedness whereby people find
  it hard to read small print
• Usually comes with age as eye loses its ability
  to accommodate
• Also corrected with converging lenses


         Demonstration at front
                   Video
• Nearsightedness and Farsightedness Video
              Contact Lenses
• Serve the same function as glasses
• Can be shaped to form a positive or negative
  meniscus
• Sit invisibly on top of the retina
• Can also be used to change eye colour
           Laser Eye Surgery
• Laser Eye Surgery Video

				
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