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MED-101 Medical Physics > THE EYE AS A COMPOUND LENS > Flashcards

THE EYE AS A COMPOUND LENS Flashcards

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1
Q
  1. What is the Gullstrand Exact eye?
A
  • it is a schematic model for the imaging rays in the eye
  • it is the most complete model
  • it has six refractive surfaces
  • there is a variation of the refractive index within the
    lens
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2
Q
  1. What is the Reduced Eye?
A
  • this is the simplest model
  • it only has one refractive interface at the cornea
  • all the distances are relative to the single refractive
    interface
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3
Q
  1. What is the Schematic Eye 1 Model?
A
  • the refractive index within the crystalline lens is
    uniform
  • it shows the surfaces of the four interfaces
  • it shows the refractive indices of the five media
  • it shows the radii of the four surfaces
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4
Q
  1. How is the refractive power of each interface given?
A
  • ignore the distances that the rays propagate
    between the refractive interfaces
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5
Q
  1. Does this working out make sense?
A
  • yes
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6
Q
  1. Does this working out make sense?
A
  • yes
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7
Q
  1. What are two thirds of the Refractive Power due to?
A
  • the cornea
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8
Q
  1. What is one third of the Refractive Power due to?
A
  • the lens
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9
Q
  1. How do we work out the Total Refractive Power of the eyes?
A
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10
Q
  1. How is the focal length found?
A
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11
Q
  1. Does this make sense?
A
  • yes
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12
Q
  1. What happens when you swim in water?
A
  • there is a change in the refractive index
  • it is changed from air to water
  • this changes the refractive power of the first interface
  • there is now a lower refractive power
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13
Q
  1. What can be said about the Refractive Power of the Cornea under water?
A
  • the refractive power of the Cornea is:
    5.19 D + (-6.11D) = -0.2 D
  • this means that there is essentially no refractive power
    under water
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14
Q
  1. What kind of images do the eyes see underwater and why?
A
  • the total refractive power of the eye is:
    -0.2 D + 22.01D = 21.90 D
  • the eye has refractive power
  • the eye sees very blurred images
  • this is because the focused image would lie beyond the
    retina
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15
Q
  1. Why can we see much better under water with goggles?
A
  • the refractive power changes
  • this is through the use of plastic
  • the images in the water are made even blurrier when
    the water is not perfectly still
  • this is because the movement of water causes local
    variations in the index of refraction
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16
Q
  1. What is Accomodation?
A
  • the ability to control the focal length of the Crystalline
    lens
17
Q
  1. What is the Crystalline Lens suspended to?
A
  • it is suspended by ligaments
18
Q
  1. What do the ligaments of the Crystalline Lens control?
A
  • the tension in these ligaments controls the curvature
    of the crystalline lens surfaces
  • this then controls the focal length
19
Q
  1. What happens when the tension in the ligaments is at a maximum?
A
  • the crystalline lens flattens
  • the focal length is at a maximum
  • a flatter lens will produce clear images of distant
    objects
  • this is proven by the Lens Equation
20
Q
  1. What happens when the ligaments are relaxed?
A
  • the crystalline lens assumes its more normal and
    spherical shape
  • a larger curvature of the crystalline lens surface
    produces a shorter focal length
  • this produces a clear image of nearby objects
21
Q
  1. What is the Far Point?
A
  • this is the maximum distance from the eye to the
    objects
  • where the objects form sharp images
  • we would like our Far Point (FP) to be infinite
22
Q
  1. What is our Near Point?
A
  • this is the nearest distance where objects are clear
  • we would like our Near Point (NP) to be 25cm
  • this is suitable for reading
23
Q
  1. What is Presbyopia?
A
  • it is the lack of appropriate accommodation
  • this means old ages
  • or old age vision
24
Q
  1. What are three causes of loss of Accomodation?
A
  1. an increase in the Crystalline lens’ Young’s Modulus
  2. a flattening of the lens
  3. decrease in the Lens’ Capsule Young’s Modulus
25
Q
  1. What is Hyperopia?
A
  • this is farsightedness
  • this is when it is difficult for a person to see things up
    close
26
Q
  1. What is Hyperopia caused by?
A
  • the focal length is too long
  • the image is focused behind the retina
27
Q
  1. What helps with Hyperopia?
A
  • a convex lens
  • the convex lens helps the eye refract the light
  • this decreases the image distance
  • this decreases the focal length
  • the image is focused on the retina
28
Q
  1. Who does Hyperopia usually affect?
A
  • it usually affects adults with weakened ciliary muscles
  • or adults with decreases lens flexibility
29
Q
  1. What is a good way to remember Hyperopia?
A
  • FAR SIGHTED
  • YOU CAN SEE FAR
  • THE RAYS FOCUS TOO FAR FROM THE LENS
30
Q
  1. What is Myopia?
A
  • it is when someone is near sighted
  • the focal length is too short
  • this causes the images of distant objects to be focused
    in front of the retina
31
Q
  1. What helps with Myopia?
A
  • a concave lens
  • the concave lens diverges the light rays
  • this increases the image distance
  • this is so that the image can be focused on the Retina
32
Q
  1. Who is nearsightedness common among?
A
  • it is common among young people
  • this is sometimes the result of a bulging cornea
  • this will refract light more than normal
  • it can also be the result of an elongated eyeball
33
Q
  1. What is a good way to remember Myopia?
A
  • NEARSIGHTEDNESS
  • MEANS YOU CAN SEE NEAR
  • THE RAYS FOCUS TOO NEAR TO THE LENS
34
Q
  1. Does this summary make sense?
A
  • yes

MED-101 Medical Physics (21 decks)

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