Optics Flashcards

1
Q

Pleochroism

A
  • Some minerals absorb different λ’s of light depending on light vibration direction
  • anisotropic minerals
    • pleochroic uniaxial - color varies btwn 2 hues
    • pleochroic biaxial - color varies btwn 3 hues
  • occurs because absorption is a function of direction–the two rays of light are absorbed differently as they pass thru the mineral
  • not related to birefringence
  • may only be observed in PPL
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2
Q

Birefringence

A
  • the property of having different indices of refraction in different directions
  • anisotropic minerals
  • occurs when the speed of light in the grain varies in different directions
  • tested by observing a mineral in XPL and with the gypsum plate in, and rotate the stage
  • birefringent minerals will display a diagnostic color range called interference colors
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3
Q

Interference Colors

A
  • Occur b/c light waves passing thru xtals can have a variety of λ’s, A’s, and phases that are affected by atomic structure n different ways.
  • in XPL there may be colors that are brighter and more pronounced than in PPL
  • They result from the interference of light rays passing thru the upper polarizer
  • determined by the difference in indices of refraction in the section
  • Diagnostic b/c different minerals display different ranges of interference colors
  • based on a formula determined by the grain orientation with respect to the vector direction of the polarized light and the thickness of the grain
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4
Q

Becke Lines

A
  • Light interacts w/the mineral grain as if were a lense, so:
    • If nmineral > nliquid then refracted rays converge
    • If nmineral < nliquid then refractd rays diverge
  • the result of 2 things:
    • the fact that minerals in thin sections act like lenses
    • internal reflection of light w/in the mineral is due to the presence of vertical grain boundaries
  • used to look at the relief and determine the index of refraction of a mineral
  • a band or rim of light visible along a grain or xtal boundary in PPL
  • best seen in intermediate or low power
  • the light becke line moves into the material of higher refractive index
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5
Q

Relief

A

Descibes the contrast btwn the mineral and its surroundings ==> as the difference btwn the RI of the liquid/mount and that of the mineral increases, the boundary btwn the two becomes more pronounced.

postive <==> negative

can you see the separation between the mineral and oil?

if the mineral has the higher RI –> positive relief

if the mineral has the lower RI –> negative relief

If the mineral and oil/mount differ substantially in their index of refraction, the mineral is said to stand out in “relief” and a dark line will appear around the mineral grain

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6
Q

Incident ray

A

the source light ray

generally splits into 2 rays when light passes from one material to another

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7
Q

Reflected ray

A

one of the two rays that split from the incident ray

the ray is reflected at the same angle that the incident ray entered the material

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8
Q

Refracted ray

A
  • one of the two rays the split from the incident ray
  • the ray that is transmitted thru the material but is bent
  • how much the refracted ray is bent depends on 2 things:
    • the difference in RI’s of the 2 materials
    • the difference in densities of the 2 materials
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9
Q

Index of Refraction

A

The ratio of the velocity (v) of light in a vacuum to the velocity in the mineral:

n = vvacuum / vmineral

Depends on chemical composition, xtal structure, and bond type in the xtal

High values of n correspond to materials tha transmit light slowly

A mineral with a high value of n suggests that it is composed of atoms with high atomic #s, or of closely packed atoms

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10
Q

Critical Angle (θCA)

A

There exists some value of θi that will make θt = 90° ==> the limiting value of θi

If the angle of incidence (θi) > the critical angle (θCA) then none of the light will escape and the entire beam will be reflected inside the xtal

Snell’s Law can be rewritten:

θCA = nt / ni

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11
Q

Snell’s Law

A

The mathematical relationship between the angle of incidence (θi) and the angle of refraction (θr).

_ sin(θ_i) _ = _ vi _ _ = nr _ _

sin(θr) vr ni

therefore:

ni x sin(θi) = nr x sin(θr)

when ni < nt then θi > θt

when ni > nt then θi < θt

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12
Q

Opaque

A

a mineral will appear dark in both PPL, XPL, and while rotating the stage in XPL

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13
Q

Isotropic

A

Having the same properties in all directions, therfore:

a mineral may appear dark or transparent in PPL

a mineral will appear dark in XPL

nothing will happen when rotating the stage in XPL

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14
Q

Anisotropic

A

Having different optical properties in differernt directions, therefore:

a mineral may appear dark or transparent in PPL

a mineral may appear dark or transparent in XPL

when rotating the stage in XPL, a mineral will display changes in color and/or shade

  • uniaxial minerals have 1 optic axis
  • biaxial minerals have 2 optic axes
  • when light passes thru a xtal’s optic axis, it appears isotropic, however the odds are low
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15
Q

Properties of Light

A
  • Different colors of light are characterized by different wavelengths (λ)
  • The intensity of a wave is proporional to its amplitude (A)
  • The electric vectors of unpolarized light vibrate in all directions perpendicular to the direction of travel
  • The electric vectors of PPL are constrained to vibrate in a single plane
  • If λ’s corresponding to all of the primary colors are present with nearly equal intensity, the light appears white.
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16
Q

Polychromatic

A

many colored

ex: white light

17
Q

Monochromatic

A

single colored

when only one λ is isolated

18
Q

Constructive Interference

A
  • when waves are in-phase
  • no energy is lost
  • waves are in-phase when their peaks and λ’s correspond, so they add to produe one wave w/twice the A
19
Q

Destructive Interference

A
  • when waves are out-of-phase, waves “consume” some or all of each other’s energy
  • whn waves are completely out-of-phase, their motions cancel and addition leads to complete loss of energy
20
Q

Dispersion

A
  • The property that RI varies with λ
  • can sometimes be see in thin section
  • Dispersion affects luster
    • high dispersion appears adamantine… ex: diamond
    • low dispersion appears dull… ex: fluorite
21
Q

Refraction

A
  • Objects appear to bend as they pass from air into water
  • Occurs when a beam of light passes from one medium to another w/a different RI
  • The beam refracts toward the medium w/the higher RI
  • A beam traveling 90° to the interface, is not refracted
22
Q

Angle of Incidence (θi)

A

The angle between the incident ray and the normal to the surface.

23
Q

Angle of Refraction (θr)

A

The angle between the refracted ray and the normal to the surface.