Brainscape's Knowledge GenomeTM

Browse over 1 million classes created by top students, professors, publishers, and experts.


See full index

Optics > experiment 8 - aberrations > Flashcards

experiment 8 - aberrations Flashcards

1
Q

what is spacial resolution limited by ?

A

spatial resolution is limited by diffraction , higher order aberrations and scattered light

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

what happens to rays close to the axis ?

A

rays close to the axis focus at the paraxial region

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

what happens as we increase aperture size?

A

rays no longer focus in the paraxial image plane

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

why do we need a spherical ideal wave front ?

A
  • the paraxial rays which originate very close to axis they appear to be part of a spherical surface which is paraxial wave front
  • only a spherical surface sends all the rays to the same point because they are simply radii of curvature and they all point to centre of sphere
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

what happens when increase ray height ?

A
  • we move from the paraxial wavefront to the real wavefront , the two no longer coincide
  • the real wavefront is displaced with respect to paraxial wavefront
  • when object is on axis the real wavefront is displaced symmetrically
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

what is the wavefront aberration function ?

A

𝛿W(x,y) maps the difference between the ideal wavefront surface ( which is always a perfect sphere ) and the actual wavefront surface( which encodes the aberrations in the lens)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

what does the distortions you have on real wavefront surface compared to ideal wavefront surface encode ?

A
  • the distortions you have on real wavefront surface compared to ideal wavefront surface encode all the aberrations
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

what should 𝛿W be for every point if system is aberration free ?

A
  • if system is free from any aberrations then 𝛿W ( wavefront )for every point should be zero
  • real wavefront matches ideal wavefront and you end up with ideal image formation
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

how do astronomers probe the universe ?

A

by getting the best possible image with optical instruments

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

what does presence of spherical aberration do in terms of spacial resolution ?

A

the presence of spherical aberrations reduces spacial resolution that can be achieved in the absence of aberration

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

what are the key parameters affecting optical aberrations ?

A

1- (β ) object field angle - if object is on axis , object field angle will be 0 and image will be on axis

2- (θ )specifies meridian in exit pupil plane

3-(h) specifies ray height in exit pupil plane

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

what are the key parameters affecting optical aberrations ?

A

1- (β ) object field angle - if object is on axis , object field angle will be 0 and image will be on axis

2- (θ )specifies meridian in exit pupil plane

3-(h) specifies ray height in exit pupil plane

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

what is spherical aberration ?

A
  • proportional to the 4th power of ray height
  • b determines how much spherical aberration you have
  • no β - which means that spherical aberration is a large on axis as off axis
  • spherical aberration independent of object field angle
  • big aperture = lot of spherical aberration
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

what is coma ?

A
  • proportional to object field angle
  • proportional to value of cosine of angle θ
  • when object is on-axis , the coma will be 0
  • absent on axis
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

what is astigmatism ?

A
  • proportional to the square of object field angle
  • proportional to β so no on-axis astigmatism
  • off axis
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

what is field curvature ?

A
  • best plane for off-axis object point is closer to the lens
  • quality of image isn’t affected by field curvature
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
17
Q

what is distortion ?

A
  • quality of image point is not affected by distortion
  • images generated are no longer geometrically similar to objects that generate those images
  • proportional to the third power of object field angle
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
18
Q

what does b1,b2,b3,b4,b5 means ?

A
  • constants that determine the size of each aberration term
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
19
Q

what are the effects of primary seidel aberrations ?

A

. expanded size for a point image ( spherical aberration ,coma and astigmatism ) as they affect the quality of the image

. curved image plane ( field curvature )

. extended images are no longer geometrically similar to the object ( distortion )

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
20
Q

what kind of effect does aberration have ?

A

aberrations are geometric effects which reveal the way rays are refracted through optical surfaces which are symmetrical with respect to the axis

21
Q

what happens if we increase ray height in spherical aberration ?

A
  • the quality of image isn’t good
  • the paraxial image rays has curved
  • the distribution of rays in paraxial image plane is no longer symmetrical
22
Q

what is the only aberration that is on-axis ?

A

spherical aberration

23
Q

explain the lab equipment designed to illustrate effects of aberration ?

A
  • light source - small dot object that is very bright
  • light goes through lens
  • single lens with variable size iris
  • goniometer which allows you to rotate the lens on horizontal and vertical plane
24
Q

what happens once we use the equipment which illustrates the effects of spherical aberration ?

A
  • initially we have small dot image
  • we start by increasing the size of iris
  • as you increase aperture size you get spreading of rays in image plane suggesting its spherical aberration
25
Q

explain spherical aberration in a concave mirror ?

A
  • in concave mirror you have a distant object with rays parallel to optical axis and all the rays are not focusing at same focal point
  • rays closer to paraxial region will focus at same point
26
Q

how to overcome spherical aberrations in mirror ?

A
  • by making the surfaces aspherical so that all rays image at the focal point
  • this is what happens in astronomical telescope
27
Q

what is the difference in lens power between the marginal and the paraxial focal planes in spherical aberration ?

A

the difference in lens power between the marginal and the paraxial focal planes is proportional to the square of ray height (h^2)

28
Q

what does the disc of least confusion divide in spherical aberration ?

A

the disc of least confusion divides the distance between marginal and paraxial foci in the ratio 1:3 for large aberration

29
Q

what happens to mean wave aberration function as pupil size increases ?

A
  • the larger the pupil size the larger the mean wave aberration function
30
Q

what happens in pupil size of less than 3mm ?

A
  • good eye is aberration free
31
Q

explain what happens in coma in the lab ?

A
  • use aperture size of a bout 2.5 cm ( medium size aperture )
  • tilt the lens by about 20deg
  • intermediate image size
    coma increases with field angle and therefor zero on axis
  • rays perpendicular to the wavefront surface generate a distribution of intensity in the image of a point that has a comet like appearance
32
Q

what is the effect of coma on wave front aberration function ?

A
  • on axis and pupil size small we end up with ideal wavefront surface
  • the real wavefront is behind the ideal wavefront
33
Q

what happens if object is on-axis for astigmatism ?

A

we have no astigmatism

34
Q

how are the tangential plane and sagittal plane produced in astigmatism ?

A
  • take a line from object point that passes through the axis so that line and the axis form a plane which is the tangential image plane
  • if we rotate point , every single point will have a different projection to axis and a different tangential and sagittal plane
35
Q

where is sagittal image line ?

A

-sagittal image line is in tangential image plane

36
Q

where is tangential image line ?

A

tangential image line is in sagittal plane

37
Q

what is difference in circle focus between tangential image line and sagittal image plane ?

A

tangential image line= circles are sharp in focus

sagittal image line = circles are blurred

38
Q

what happens to line that passes through axis the further out you go ?

A
  • the line that passes through axis becomes more and more blurred the further out you go because the amount of astigmatism is proportional to the square field angle
  • further out you are = the larger object field angle
39
Q

which focal line is closer to lens in astigmatism ?

A
  • tangential focal line which is in sagittal plane is closer to the lens
40
Q

what happens if aberration is zero in astigmatism ?

A
  • rays are imaged as a line ( sagittal focal line )
41
Q

where is best focus ?

A

between sagittal and tangential image plane

42
Q

describe astigmatism as wavefront aberration ?

A
  • the tangential or meridian plane (θ=0) has the largest aberration whilst the sagittal plane (θ=90) has no aberration
  • the dioptric difference in power between the tangential and sagittal focal lines increases with the square of the field
  • in symmetrical optical systems astigmatism is absent on axis when β=0 deg
  • astigmatism in the eye is often caused by a significant difference in the curvature of the cornea in one meridian
43
Q

what is astigmatism cause by ?

A
  • ## caused by difference in power from rays originating in one meridian compared to other meridian
44
Q

what does we have on axis in astigmatism ?

A

we have astigmatism caused by change in curvature of corneal surface in one meridian with respect to orthogonal meridian

45
Q

why are we not concerned with oblique astigmatism in the eye ?

A

in human eye we are not concerned with oblique astigmatism because its in the periphery of visual field where our visual acuity is poor

46
Q

how do we correct astigamtism?

A

by using toroidal lenses which have specific power which cancel out increase power in astigmatic meridian

47
Q

describe field curvature as a wavefront aberration ?

A
  1. field curvature represents a variation in focusing distance that varies with the square of the field
  2. field curvature is an advantage given the shape of imaging surface
  3. field curvature increases with the square of the object field angle and ray height
  4. field curvature can be combined with astigmatism to ensure that the best astigmatic image plane forms a flat surface
48
Q

what are the two types distortion?

A
  • image with negative or pincushion distortion

- image with positive or barrel distortion

49
Q

when do monochromatic aberration arise ?

A

arise as a natural consequence of refraction and reflection in symmetrical optical systems

Optics (34 decks)

Brainscape helps you reach your goals faster, through stronger study habits.
© 2024 Bold Learning Solutions. Terms and Conditions