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Optics and Photonics > Optical Instrumentation > Flashcards

Optical Instrumentation Flashcards

1
Q

What are some examples of third order/monochromatic aberrations?

A
  • spherical aberration
  • coma
  • astigmatism
  • curvature of field
  • distortion
  • chromatic aberration (refracting elements)
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2
Q

What causes aberrations?

A

Imperfections in optical elements and rays hitting the elements beyond the paraxial approximation.

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3
Q

What is a spherical aberration?

A

Image points formed by rays from the same object point will appear to come from different positions due to reflection/refraction from different points of the imaging element.

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4
Q

How can you rectify spherical aberrations?

A

Stopping down the system to restrict the cone of rays or use a combination of positive and negative lenses to cancel out the aberration.

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5
Q

What is chromatic aberration?

A

Due to different wavelengths of light experiencing different refractive indices passing through an element, the focal length becomes wavelength dependent.

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6
Q

How can we reduce chromatic aberration?

A

Use mirrors or multiple refractive elements of opposite powers.

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7
Q

Reflecting telescopes use …

A

mirrors

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8
Q

Refracting telescopes use…

A

lenses

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9
Q

What type of telescope uses both mirrors and lenses?

A

catadioptric

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10
Q

What type of telescope is the Keplerian/astronomical and what type of image does it produce (how)?

A
  • refracting telescope
  • uses two lenses and produces inverted image
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11
Q

What type of telescope is the Galilean and what sort of image does it produce (how)?

A
  • refracting telescope
  • uses a negative focal length eyepiece lens to produce an upright image
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12
Q

What allows a real image in the object plane to be produces by Keplerian and Galilean telescopes?

A

Large objective lens collecting ~ parallel rays from distant objects

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13
Q

How do telescopes often correct for chromatic aberration?

A

Making the objective lens a doublet

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14
Q

The intermediate image for the eyepiece is the image formed by the objective. What type of image is it for Keplerian and Galilean?

A

Real for Keplerian
Virtual for Galilean

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15
Q

What is the formula for angular magnification for a telescope?

A

M = fo/fe

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16
Q

What sign of M (angular magnification) gives an upright image?

A

negative

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17
Q

What is the formula for telescope length?

A

L = fo + fe

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18
Q

What is the exit pupil for telescopes and where should it be?

A

The image of the entrance pupil (objective lens) in the ocular)
It should be far enough from eyepiece for comfortable eye relief

19
Q

Where is the exit pupil Galilean

A

behind eyepiece

20
Q

Where is the exit pupil Keplerian

A

just outside ocular

21
Q

Where would a reticule be placed for an astronomical telescope?

A

in the real intermediate image

22
Q

What is the formula for linear magnification of a telescope?

A

me = Dex/Dobj = -fe/fo = 1/M

23
Q

Why are reflecting telescopes favoured for larger installations?

A
  • don’t require large heavy lenses
  • maintain optical quality
  • remove chromatic aberration
24
Q

Name the 3 basic reflection telescopes

A

Newtonian, Cassegrain, Gregorian

25
Q

How does a Newtonian telescope work?

A
  • parabolic focusing mirror, all rays to focal point
  • plane mirror intercepts rays, diverting them to a point near the eyepiece
  • parabolic mirror removes spherical and chromatic aberrations
26
Q

How does the Cassegrain telescope differ from the Newtonian?

A
  • it uses a hyperbolodial mirror instead of a parabolic mirror
  • this focusses light through a hole in the primary mirror
  • allows perfect imaging between primary and secondary mirrors
27
Q

What type of secondary mirror does the gregorian telescope use?

A

concave ellipsoid

28
Q

Describe how the Schmidt telescope works

A
  • catadioptric
  • refractive component is a correcting plate ensuring spherical aberration from the concave mirror used to form the image is minimised
  • this modifies the optical path
  • it is a modification to conventional reflection telescope
29
Q

What differs about the rays coming into a telescope and a microscope?

A

the rays coming into a microscope are coming from nowhere near infinity

30
Q

where does a microscope form real and virtual images?

A

a real image is formed within the microscope tube
intermediate image is just inside the eyepiece focal length
the observer sees an inverted, magnified, virtual image

31
Q

How can you give a hard edge to the field of view of a microscope?

A

Place an aperture by the intermediate image to act as a field stop

32
Q

What is the formula for the angular magnification of a microscope?

A

M = 25(cm)/feff(cm) = 25(fe + fo - d)/fofe

33
Q

What is the image to object distance ratio for a microscope?

A

so’/so = (d-fe-fo)/fo
where so’=d-fe (d is dist between lenses)

34
Q

(in words) what is the total magnification for a microscope?

A

the product of angular magnification of eyepiece at infinity and linear magnification of objective

35
Q

What is the formula for total magnification of a microscope?

A

M = (so’/so)(25/fe) = -(25/fe)(L/fo)
standardised L=16c,

36
Q

What is the lateral magnification?

A

m = hi/ho = L/fo
where L is dist between objective image and second focal length

37
Q

How are modern microscopes infinity corrected?

A

They incorporate a tube lens between the objective and eyepiece to create infinity space between tube and objective

38
Q

What happens to the focal length and diameter of the objective as the magnification increases?

A

They decrease

39
Q

(in words) what is the numerical aperture

A

The ability of a lens to collect rays

40
Q

What is the formula for NA

A

NA = nsinθ where θ is the half angle of the cone of rays from an object to the edge of the lens

41
Q

What is the maximum value for NA in air

A

1

42
Q

How does a microscope system overcome the limit of the numerical aperture?

A

using an immersion oil improves the NA, matching to the refractive index of the coverslip to increase NA by noil

43
Q

What is total internal reflection fluorescence microscopy?

A

images a very thin section of a sample above coverslip, requires very high NA

Optics and Photonics (20 decks)

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