interference colors/birefringence and properties related to refractive index Flashcards
Tell me the properties of isotropic minerals
- light isn’t split, it keeps the same velocity in all directions -chem bonding same in all directions (light finds same electronic interaction in all directions)
- λ is the same in all directions bc frequency is constant (so color will remain the same with rotation)
- if iso is strained, they may become aniso
explain the calcite experiment
refer to slide
- calcite splits the light into two rays. adding another polarizer will block out one ray
- high bifringence gives calcite the double refraction
tell me about the properties anisotropic minerals
- light at 2 diff directions travels at different velocities (can interfere destructively or constructively)
- when the two light waves (that are different as mentioned above) pass through the upper polarizer, the resultant wave has different λ and diff velocity which produces INTERFERENCE COLOR
- the amount which the waves are out of phase is RETARDATION (∆), which depends on: .
- n (refractive index) of the two perpendicular vibe directions
- orientation of crystal
- thickness of crystal ∆ = thickness x (ns-nf)
Birefringence color chart (= Michel- Levy chart) d = nhigh - nlow REFER TO SLIDE\
-velocities depend on refractive indices so the mineral splits the light into 2 (n1 doesn’t =n2)
Which is retardation. Interference bw the two lights will determine the interference color (inphase or out of phase)
-interference color dependent upon retardation
****-delta=d(n1-n2)-WHAT DOES THIS MEAN?? (bifringence) PAN
-amount the rays are out of phase is retardation
tell me about Michel-Levy Chart
1st order retardation = 0-550 mm
2nd order retardation= 550-1100 mm
3rd order retardation= 1100-1650 mm
Higher order colours are paler 550 mm – average wavelength of visible light Indicate the order of the interference colour
what happens when Different grains of same mineral are in different crystallographic orientations:
some crystals display max. birefringence, others intermediate birefringence, others min. birefringence or extinction.
tell me about mineral properties: index of refraction
refer to slide
what is refractometry?
-determination of the refractive index , This method relies on having immersion oils of known refractive index and comparing the unknown mineral to the oil.
refer to slide
- it is how refractive indices are measured
- is used in gemology because no destruction test is needed
- refractive indices determine the difference in minerals by comparing known refractive index of the oil to the unkown refractive index of the mineral (quantitatively)
- beta light determines the refractive index qualitatively
- determined by the Becke Line test
what is relief and the different kinds of relief?
-relief is the degree which a mineral contrasts other surrounding materials
Strong relief: indices of the mineral and surrounding medium differ by greater than 0.12 RI units
moderate relief: mineral does not strongly stand out, but is still visible, indices differ by 0.04 to 0.12 RI units.
low relief: mineral does not stand out from the mounting medium, indices differ by less than 0.04 RI units from each other.
tell me about the properties of relief
- measure of relative difference in n bw mineral grain and its surroundings
- determined visually by PPL
- estimates n
- minerals may exibit + or
- relief: pos: n min>n epoxy ex. garnet 1.76 neg: n min
- strong relief-refractive index contrast is large
- moderate relief-refractive index contrast is small but still can be seen
- low relief-refractive index has no contrast
- THE VALUES OF EACH TYPE ARE ON SLIDE. KNOW THESE
what causes relief?
refer to slide
- difference in velocity and how the light is being refracted
- use Becke line test to determine what was said above
what is a Becke line?
is a band of light visible outside the grain
refer to slide
what is lens effect?
lens effect: Mineral fragments thinner on edges = act as lenses If RI of mineral is higher than surrounding it acts as a converging lens and light is concentrated toward the centre of the grain internal reflection effect:
Edges of grains must be vertical at some point
Moderately converging light is either refracted or internally reflected at the grain boundary
Results in light concentration in the material with higher RI (n)
tell me more about Becke lines
refer to slides
PAN
tell me about what happens when you move towards and away from the mineral with the becke line
when you move AWAY, the becke line moves INTO the mineral when you move CLOSER, the becke line moves OUTSIDE the mineral
refer to slide
what happens when light is perpendicular or // to c axis in anisotropic minerals an how does it affect to bifringence, retardation and intererence
PAN or recording!
what is retardation? tell me about it
- delta
- amount of rays out of phase in anisotropic minerals
- rays perpendicular to c axis is min retardation
- rays // to c axis is max retardation
- retardation =thickness(ns-nf)
- (ns-higher refractive index, nf-low refractive index)
- net effect is determined by michel levy chart, which shows that color is overall affected by amount of retardation
1st order-sharp colors, subsequent orders become more pale
-we have to know how to recognize 1st, 2nd, 3rd order retardation
