STRUCTURE OF THE EYE Flashcards
1
Q
- 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
2
Q
- Through which part of the eye does light enter?
A
- it enters through the Cornea
3
Q
- Through which part of the eye is light directed through?
A
- light is directed through the Anterior Chamber
4
Q
- What happens at the Retina?
A
- the formation of the image happens here
5
Q
- What is the average radius of the eye?
A
- it has a radius of 12nm
6
Q
- 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
7
Q
- 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
8
Q
- 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
9
Q
- What is the diameter of the pupil?
A
- it can vary from 1.5 to 10 mm in diameter
10
Q
- What controls the diameter of the pupil?
A
- the opposing pair of smooth muslces
11
Q
- What can be said about the position of the Crystalline lens?
A
- it is suspended
- it rests on the Posterior Surface of the Iris
12
Q
- 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
13
Q
- Why can the Crystalline lens be described as Onion Like?
A
- it has new layers that grow over the older layers as we age
14
Q
- What 2 factors is the formation of the image on the Retina determined by?
A
- THE INDICES OF REFRACTION
- of each eye component
- that the light passes through
- THE SHAPE OF THE SURFACES
- of these elements
15
Q
- 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
16
Q
- How many rod cells are there in one Retina?
A
- 120 million
17
Q
- What are five characteristics of the Rod Cells in the Retina?
A
- they are highly sensitive
- they have low spatial accuracy (detection of shape)
- they are more of them in the periphery of the Retina
- they are sensitive to dim lights
- they are not sensitive to colours
18
Q
- What is the sensitivity of the Rod Cells?
A
- their sensitivity peaks to nearly 500 nm
19
Q
- What does vision through only rod cells look like?
A
- everything would be seen in various shades of gray
20
Q
- What two types of vision do Rod Cells contribute towards?
A
- Night Vision
- Peripheral Vision
21
Q
- 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
- How many cone cells are there per Retina?
A
- 6.5 million cone cells
23
Q
- What are three characteristics of Cone Cells?
A
- they have a low sensitivity
(they have a sensitivity that is 1000 x lower than rods) - they have high spatial awareness
- they are concentrated in the Fovea
24
Q
- What are the 3 types of Cone Cells?
A
- Blue Cones
- they have a spatial sensitivity that peaks near 445nm - Green Cones
- they have a spatial sensitivity that peaks near 535nm
- Red Cones
- they have a spatial sensitivity that peaks near 670nm
25
Q
- What are sharp vision and colour vision due to?
A
- they are due to cones
26
Q
- What does damage to the fovea lead to?
A
- images will be viewed as fuzzy
27
Q
- 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
- 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
- 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
- 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
- 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
- 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
- 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
- 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
- What kind of light does the Crystalline lens absorb?
A
- it absorbs most of the light from 300 to 400 nm
36
Q
- Are there any important sources of absorption in the Visible Spectrum?
A
- no
