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Optometry Year 1 > OV1001 Optics > Flashcards

OV1001 Optics Flashcards

1
Q

What is light?

A

A form of radiant energy

Wave particle duality

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2
Q

What are the rules of geometrical optics

A
  • Light travels in the form of rays that are coming from a source of light
  • A ray of light travels in a straight line until it meets an optical boundary
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3
Q

What happens to light rays as it passes a diffuse surface?

A

Light rays scattered in all directions

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4
Q

What happens to light as it travels through a more optically dense medium?

A

It travels more slowly

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5
Q

What do closer wavefronts within glass mean?

A

Indicates light is travelling at a slower speed

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6
Q

What happens to wavefront as it enters a more optically dense medium?

A

Wavefronts has been tilted because of the uneven slowing down

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7
Q

What does a higher refractive index mean?

A

More dense material

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8
Q

What can prisms help treat?

A

Muscular imbalance

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9
Q

What happens when objects are viewed through a prism?

A

They are displaced towards the apex of the prism

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10
Q

What does the amount of displacement depend on?

A

Angle of the apex

Thickness of prism

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11
Q

If a prism is thicker what does that mean?

A

Stronger amount of correction

Displaces objects to a greater degree

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12
Q

What is dispersion?

A

White light refracting into different colours when shone through a prism

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13
Q

What happens to rays as they go from a low refractive index to a high one?

A

They refract towards the normal

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14
Q

What is the ideal optical system?

A

All rays converging to a single point

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15
Q

What does the paraxial approximation do?

A

Ignores rays that refract too far

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16
Q

Key feature for n=c/v

A

Always greater than 1

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17
Q

What is the principal of reversibility ?

A

If we swap the location of object and image, the system behaves in the same way

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18
Q

What is vergence?

A

Measure of how much a set of rays are diverging or converging

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19
Q

How do you know if an optical surface has power ?

A

If it has the ability to change the vergence upon refraction

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20
Q

What does it mean if you get reflection instead of refraction? (TIR)

A
  • Light is travelling from a medium with higher refractive index to a medium with lower refractive index (i.e. n > n0)
  • The angle of incidence i is greater than the critical angle of incidence, ic (i.e. i > ic).
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21
Q

Define centre of curvature?

A

The centre of curvature of a spherical surface is a point representing the centre of the sphere the surface would be part of, had the whole sphere been present

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22
Q

Define the first focal point

A

The point on the optical axis such that the object rays that meet or originate from this point will refract parallel to the optical axis

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23
Q

Define the second focal point

A

The point on the optical axis where the image rays meet if the object rays are parallel to the optical axis

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24
Q

Why is the thin lens approximation used

A

Allows us to ignore this change in vergence due to the displacement through the lens

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25
Q

What causes focal points to be equidistant from the lens?

A

Optical media on either side of the lens is the same

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26
Q

Where is an image formed if the object is placed at the first focal point?

A

Infinity

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27
Q

How do you achieve a vergence of 0 in a focimeter?

A

Change vergence arriving at test lens

Move target until parallel rays are achieved

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28
Q

What are the conditions for H’ to e equivalent to a thick lens?

A
  • Image it produces must be identical to that of a thick lens
  • Same object rays
  • Same image rays
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29
Q

What properties does a specular mirror have?

A

Flat

Does not change vergence of the beam

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30
Q

For transparent optical surfaces what does the values of R and T depend on?

A

The refractive idex change of he surface

Angle of incidence

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31
Q

Where are F and F’ for mirrors?

A

Coincident

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32
Q

What sign is given for the refractive index of image space?

A

Negative

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33
Q

What are purkinge images

A

The virtual image produced due to the reflected light from the first surface of the eye

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34
Q

What are purkinge images used to assess

A

Curvature
Separations of the surfaces in the eye
Eye tracking

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35
Q

What does purkinge images depend on

A

Refractive indices of surfaces - larger difference larger reflection

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36
Q

What does diffuse reflections in the eye allow?

A

Visualise internal structures of the eye

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37
Q

What do two lenses need to have to have identical cardinal points

A

Same:

  • material
  • radius of curvature
  • thickness
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38
Q

How will the size of the lens affect the image produced?

A

Larger lens allows more light through the optical system giving a brighter image

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39
Q

What happens when you use a stop?

A

You reduce the aperture of the lens

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40
Q

Where is the stop located

A

Between the lens and image

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41
Q

What does the asymmetric use of lens introduce

A

Aberrations

Imperfections which degrade the image

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42
Q

What is a lens

A

An aperture that is defined by the diameter

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43
Q

What does a lens with a larger diameter have

A

Larger aperture

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44
Q

Define Aperture

A

Any clear region of optical surface through which light can be transmitted

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45
Q

Define aperture stop

A

The aperture of the optical system as a whole.

The largest cone of light that can be transmitted by the optical system

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46
Q

What does a chief ray show

A

The largest beam of light passing through aperture

stop

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47
Q

Define chief ray

A

Ray that passes through centre of aperture stop

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48
Q

What do extended marginal rays show

A

The size of the entrance pupil

The largest cone of light leaving the object that can go through the system

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49
Q

What is a field stop

A

A structure that limits the field of view of the system

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50
Q

Define field of view

A

Extended field that can be imaged by the camera

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51
Q

What happens when the field stop is not at the image plane

A

Image is not as bright and abberated due to asymmetric use of the beam
Gradual cut off in image

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52
Q

What determines aperture stop

A

Axial rays

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53
Q

What determines field stop

A

Off-axis rays

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54
Q

What happens in a direct opthalmoscope?

A

Some of the light leaving the patients pupil is lost and does not make it through the sight hole

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55
Q

What can you do increase the field of view in an opthalmoscope?

A

Move closer to the patient so you can get a larger view of the retina

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56
Q

What happens in an indirect opthalmoscope?

A

Px pupil is imaged onto examiners pupil using high powered lens

E and E’ is on the same plane

57
Q

What happens when the examiners views the px more off axis in a direct opthalmoscope?

A

only part of the beam will go through the sight hole

58
Q

What observations cannot be explained through geometrical optics

A

Light doesn’t always travel in straight lines

Light can interact with itself, not just with matter

59
Q

Define amplitude

A

The height between the peak and trough

Represents how much energy is present in the wave

60
Q

Define Phase

A

The position of the wave if two waves of the same wavelength are displaced w/ respect to each other

61
Q

What does a light wave need to oscillate?

A

Electromagnetic field

62
Q

What happens an object is emitting light ?

A

It creates an electric and magnetic field that coupled together and oscillate in sync

63
Q

What happens to the wavefronts of light as it travels through a denser material ?

A

It becomes distorted

64
Q

What happens in unpolarised light?

A

Electric field fluctuates in all directions

65
Q

What happens in polarised light?

A

Electric field in only one direction

66
Q

Where is polarised light used

A

Photo elastic stress analysis

Light microscopy

67
Q

What does ‘by a fringes mean’

A

The material has a refractive index that depends on the polarisation and propagation direction of light

68
Q

Where will light reflect

A

At the first surface at the active reflection coating

Surface of the glass

69
Q

What will happen if the thickness of the medium is correct for a light ray passing through a medium?

A

Ray will end up with two waves out of phase, peak coinciding with a trough and will cancel each other out

70
Q

Define diffraction

A

Property of light causing wavefronts to no longer be plane but slightly bent on the extremes on the wavefront

71
Q

What causes diffraction

A

When light squeezes through an aperture

Light deviates from the straight line trajectory causing a bend in the wavefront

72
Q

What does diffraction depend on

A

Size of the aperture

73
Q

What happens to the effect of diffraction as the beam travels further away from the aperture

A

Effects becomes greater

Starts to affect the centre of the wavefront

74
Q

What wavefronts give perfect objects

A

Spherical wavefronts

75
Q

What is an airy disc

A

Image of a point object due to the effect of diffraction

76
Q

What does a smaller aperture produce

A

Larger airy disc

More diffraction

77
Q

What is Point spread function

A

The image if any optical system gives when an object is a point

78
Q

What can the shape of PSF tell us

A

The diffraction and aberrations present in an optical system

79
Q

What is a good measure of how good our image is

A

Deviation between reference and actual wavefront

80
Q

What is a measure of aberrations

A

Distance along optical axis between both meridians

81
Q

What makes up PSF

A

Combination of the effect of aberration and diffraction

82
Q

What causes aberrations

A

Due to the imperfect shape of the optical system

83
Q

What happens to PSF when aperture is at largest

A

PSF is dominated by aberrations with a very asymmetric PSF

84
Q

What happens to PSF when aperture is very small

A

Non-paraxial rays removed
No aberrations
Only diffraction present

85
Q

What does a larger PSF mean

A

The more blurred an image is

86
Q

When are small/paraxial approximations valid

A

When ray height (h) is small compared to the radius of curvature of the surface (r)

87
Q

Define ideal image formation

A

Lens produces a perfectly spherical wavefront surface

When magnification is the only parameter needed to restore image to original object

88
Q

What is the wavefront aberration function

A

Corresponding difference between the paraxial and the real wavefront

89
Q

What does it mean when the wavefront aberrations to be zero

A

System is free of aberrations

Real wavefront surface is matched the paraxial wavefront for every ray height

90
Q

Define the paraxial optics region of an optical system

A

Rays that pass close to the centre of the lens and originate from object points that are close to the axis of the lens

91
Q

What are the principal parameters that affect what we see

A

Stimulus size
Stimulus contrast
Retinal sensitivity to contrast

92
Q

What is stimulus size

A

Its angular subtense at the eye

93
Q

What is stimulus contrast

A

The luminace difference between the optotype and the adjacent background

94
Q

What is weber contrast

A

Iuminance difference w/ respect to the background

95
Q

What is michelson contrast

A

Increase/decrease in the contrast w/ respect to the average luminance

96
Q

Define MTF

A

measure of the ability of an optical system to transfer various levels of detail from object to image.

97
Q

What happens when light levels drop

A

Retina illuminance decrease
Retina is less able to resolve fine detail
Results in poorer acuity
Poorer functional contrast sensitivity

98
Q

What are the conditions for MTF

A

Ideal eye
No aberrations
Only limited by diffraction

99
Q

What are the advantages for using contrast sensitivity

A
  • Sensitive to residual higher order aberrations, diffraction and increased scattered light
  • Can reveal presence of subclinical retinal disease
100
Q

What does a high spatial frequency mean

A

Large number of cycles per degree of visual angle

Width of bright and dark bars becomes very small

101
Q

What does a forced choice procedure allow

A

Improved accuracy

102
Q

What happens to mesopic Px when using contrast optoptypes

A

More affected by the increased scatter within the optics of the eye

103
Q

Why do we measure Functional contrast sensitivity?

A

VA is not sensitive to early changes in the optics of the eye
VA not very sensitive to early structural changes in the retina caused by disease

104
Q

Advantages of measuring FCS

A

Measurement variable - stimulus contrast
Fixed size optotype
One can use both positive and negative contrast polarity

105
Q

What imposes the ultimate limit on the resolving power of any optical system

A

Likely to get diffraction around edge of aperture

Causes formation of airy disc

106
Q

Define airy pattern

A

Diffraction pattern caused by a circular aperture

107
Q

What does the rayleigh criterion require

A

The separation between two adjacent object points to produce an image separation equal to p (the radius of the first dark ring of the airy pattern)

108
Q

What happens to the resulting power of the objective lens as the diameter of the aperture increases

A

Resulting power increases

Angle that can be resolvable becomes smaller

109
Q

What key parameters affect aberration

A

Ray height
Meridian angle
Object field angle

110
Q

What determines ray height

A

Size of the aperture

111
Q

How is the object field angle formed

A

Formed when dealing with extended off axis objects

112
Q

What are the effects of primary seidal aberrations

A

Expanded size for a point image
Curved image plane
Extended images are no longer geometrically similar to the objects

113
Q

Why is spherical aberrations absent on axis

A

It doesn’t depends on the object field angle

114
Q

What are the two image planes for spherical aberrations

A

Paraxial and marginal

115
Q

What does the disc of least of confusion correspond to

A

The best image plane

116
Q

What does the disc of least of confusion do

A

Divides between marginal and paraxial foci in the ration 1:3 for large aberrations

117
Q

What is the difference in lens power between the marginal and paraxial focal planes proportional to

A

The square of ray height

118
Q

What happens to coma when object field increase

A

Larger amounts of coma and more asymmetric distribution of rays

119
Q

What do rays perpendicular to the wavefront surface generate

A

A distribution of intensity in the image of a point that as a comet like appearance

120
Q

What is astigmatism in the eye caused

A

Due to the significance difference in the curvature of the cornea in one meridian.
Astigmatic like effects will be observed for on axis objects

121
Q

What happens to an object with circular symmetry in the object plane

A

It will be images with good spatial resolution while a line that passes through the axis will be more blurred

122
Q

What does field curvature represent

A

A variation in focusing distance that varies with the square of the field

123
Q

Why is field curvature an advantage in the eye

A

Given the shape of the imaging surface

124
Q

Why is a negative field combine with astigmatism

A

To ensure that the best astigmatic image plane forms a flat surface

125
Q

How do you minimise the effects of field curvature

A

Use a curved image surface
Use a combination of positive and negative lenses
Introduce appropriate amounts of astigmatism

126
Q

What does distortion represent

A

A displacement of focus for each image point in a radial direction that increases with the third power if the field

127
Q

How does monochromatic aberrations arise

A

Due to the natural consequence of refraction and reflection in symmetrical optical systems

128
Q

What seidal aberrations affect the quality of the image by distorting the shape of the wavefront

A

Spherical aberrations
Astigmatism
Coma

129
Q

What seidal aberrations affect the position of the image without distorting the PSF

A

Field Curvature

Distortion

130
Q

What aberrations cause image blurring and irretrievable loss of spatial detail

A

Astigmatism

Spherical aberrations

131
Q

How do the refractive indices of most transparent material used in optical instruments vary in the same way

A

All show an increase in refractive index as the wavelength is decreased

132
Q

What is v value

A

quantifies the change in power in a thin lens as result of dispersion

133
Q

Why are the specific wavelength chosen for spectral lines

A

Easily produce from low pressure discharge lamps

They cover electro-magnetic spectrum to which the eye is sensitive

134
Q

What is spectral luminous function of the eye

A

The sensitivity of the eye to different wavelength

135
Q

What does longitudinal chromatic aberration in the human eye cause

A

Shifts in the image plane position with wavelength

136
Q

What can a negative lens used to correct myopia also cause

A

A decrease in the chromatic aberration in the eye

137
Q

What structure in the eye acts as ‘effective’ field stop for visual acuity

A

the entry pupil of the eye

138
Q

What are the principal factors that affect the sharpness of the image and the resolving power of the lens?

A

High order aberrations
Scattered light
Diffraction

139
Q

The Exit Pupil, E0

A

the image of the aperture stop seen from the exit of the system. It is therefore the image of the aperture stop formed by all the optical elements behind the aperture stop.

Optometry Year 1 (5 decks)

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