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MED-101 Medical Physics > STRUCTURE OF THE EYE > Flashcards

STRUCTURE OF THE EYE Flashcards

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1
Q
  1. Provide the names for the parts of the eye
    labelled 1-12.
A

1= Ciliary Body and Muscle
2= Iris
3= Pupil
4= Lens
5= Cornea
6= Anterior Chamber
(this is filled with aqueous humour)
7= Sclera
8= Choroid
9= Retina
10= Macula
11= Optic Nerve
12= Vitreous Body

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2
Q
  1. Through which part of the eye does light enter?
A
  • it enters through the Cornea
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3
Q
  1. Through which part of the eye is light directed through?
A
  • light is directed through the Anterior Chamber
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4
Q
  1. What happens at the Retina?
A
  • the formation of the image happens here
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5
Q
  1. What is the average radius of the eye?
A
  • it has a radius of 12nm
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6
Q
  1. Why is the Cornea transparent?
A
  • it is transparent because it is uniform in structure
  • it is avascular
    (it has a lack of blood vessels)
  • it is relatively dehydrated

NB:
- the cornea has blood vessels present in the extreme
periphery

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7
Q
  1. What covers the Cornea?
A
  • it is covered by a 7 - 10 μm thick layer of tears
  • this smoothes over Optical Irregularities
  • it supplies oxygen to the cornea
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8
Q
  1. What is the average radius of Curvature of the Anterior Surface of the Cornea?
A
  • it is about 7.8 mm in the central region
  • people have a variation of this of about +/- 0.4 mm
  • it is flatter in the periphery
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9
Q
  1. What is the diameter of the pupil?
A
  • it can vary from 1.5 to 10 mm in diameter
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10
Q
  1. What controls the diameter of the pupil?
A
  • the opposing pair of smooth muslces
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11
Q
  1. What can be said about the position of the Crystalline lens?
A
  • it is suspended
  • it rests on the Posterior Surface of the Iris
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12
Q
  1. What is the thickness and the diameter of the
    Crystalline lens?
A
  • it is about 4 mm thick
  • the diameter of the lens is 9 mm
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13
Q
  1. Why can the Crystalline lens be described as Onion Like?
A
  • it has new layers that grow over the older layers as we age
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14
Q
  1. What 2 factors is the formation of the image on the Retina determined by?
A
  1. THE INDICES OF REFRACTION
    • of each eye component
    • that the light passes through
  2. THE SHAPE OF THE SURFACES
    • of these elements
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15
Q
  1. What are the focusing elements in the eye?
A
  • the Cornea
  • the Crystalline Lens
  • the Cornea performs about ⅔ of the focusing
  • the Crystalline lens performs the remaining ⅓
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16
Q
  1. How many rod cells are there in one Retina?
A
  • 120 million
17
Q
  1. What are five characteristics of the Rod Cells in the Retina?
A
  1. they are highly sensitive
  2. they have low spatial accuracy (detection of shape)
  3. they are more of them in the periphery of the Retina
  4. they are sensitive to dim lights
  5. they are not sensitive to colours
18
Q
  1. What is the sensitivity of the Rod Cells?
A
  • their sensitivity peaks to nearly 500 nm
19
Q
  1. What does vision through only rod cells look like?
A
  • everything would be seen in various shades of gray
20
Q
  1. What two types of vision do Rod Cells contribute towards?
A
  1. Night Vision
  2. Peripheral Vision
21
Q
  1. What is the diameter of rods?
A
  • they have a diameter of 2 μm
  • they are far away from the Fovea
  • they become more widely spaced
  • many rods are connected to the same nerve fibre
  • this decreases the visual acuity in the outer portions of
    the retina
    ACUITY = sharpness of vision
22
Q
  1. How many cone cells are there per Retina?
A
  • 6.5 million cone cells
23
Q
  1. What are three characteristics of Cone Cells?
A
  1. they have a low sensitivity
    (they have a sensitivity that is 1000 x lower than rods)
  2. they have high spatial awareness
  3. they are concentrated in the Fovea
24
Q
  1. What are the 3 types of Cone Cells?
A
  1. Blue Cones
    - they have a spatial sensitivity that peaks near 445nm
  2. Green Cones
    • they have a spatial sensitivity that peaks near 535nm
  3. Red Cones
    • they have a spatial sensitivity that peaks near 670nm
25
Q
  1. What are sharp vision and colour vision due to?
A
  • they are due to cones
26
Q
  1. What does damage to the fovea lead to?
A
  • images will be viewed as fuzzy
27
Q
  1. What are Photons?
A
  • they are when light acts like little packets of light
  • it is one form of light’s dual nature
  • the other form is waves
28
Q
  1. How do we calculate the energy of a photon?
A

E = hf
E = hc / λ

E = energy of the photon
h = Plank’s Constant
f = frequency of the light
c = speed of light in a vacuum
λ = wavelength of a light

29
Q
  1. When does Absorption happen?
A
  • when photons have energy in ranges
  • this energy can be absorbed
  • it is absorbed by the photosensitive molecules in these
    cells
  • these photosensitive molecules are called Rhodopsin
30
Q
  1. What percentage of visible light incident on the eye reaches the retina?
A
  • 50% of visible light
  • this has a wavelength of 400-700nm
  • this visible light will form an image
31
Q
  1. Why does 100% of visible light incident not reach the retina?
A
  • this is due to losses
  • these loses are due to:
    • reflection of light
    • absorption
    • scattering

NB:
- most of the loss comes from scattering in the eye

32
Q
  1. Does the scattered light contribute to the desired image even if it does hit the retina?
A
  • no
  • this light may be refracted in a random direction
  • the image will not be clear
33
Q
  1. What kind of losses happen within the Retina before the light hits the rods and the cones?
A
  • there are losses due to absorption and scattering
  • this is also due to the transmission through the Retina
34
Q
  1. How do we write the expression for the portion of light that reflects from an interface between two media?
A
  • n1 and n2 are the refractive indices
  • the refracted fraction at the air and the cornea
    interface is around 2%
35
Q
  1. What kind of light does the Crystalline lens absorb?
A
  • it absorbs most of the light from 300 to 400 nm
36
Q
  1. Are there any important sources of absorption in the Visible Spectrum?
A
  • no

MED-101 Medical Physics (21 decks)

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