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Refraction/isotropic minerals

Refraction/isotropic minerals. refraction. Light that is bent Index of refraction (n): how effective a material can bend light N medium = V vacuum /V medium Most minerals 1.4>n<2.0 High index = low velocity Nwater = 1.33 Nglass = 1.45. Other relationships of light. F = v/ 

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Refraction/isotropic minerals

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  1. Refraction/isotropic minerals

  2. refraction • Light that is bent • Index of refraction (n): how effective a material can bend light • Nmedium = Vvacuum/Vmedium • Most minerals 1.4>n<2.0 • High index = low velocity • Nwater = 1.33 • Nglass = 1.45

  3. Other relationships of light • F = v/ • Since f is constant v1/1 = v2/2 • So v1 / v2 = 1 /2 • Since v1=vvacuum/n1 • Then n1/ n2 = 2 /1

  4. Example: • N(oil) = 1.54, v(air) = 3*10^8 m/s • V(oil) = v(air)/n(oil) = 1.95*10^8 m/s • oil/nair = air/noil • oil =589nm/1.54 = 383 nm

  5. Snell’s law • n1sin(angle1) = n2sin(angle2) • Light bent towards the normal in the higher refractive index

  6. Light always refracted from low to high-index material • Light from high to low-index mineral can have a critical angle • Critical angle: refraction angle is 90° • Beyond this angle, total internal reflection

  7. Light always refracted from low to high-index material • Light from high to low-index mineral can have a critical angle • Critical angle: refraction angle is 90° • Beyond this angle, total internal reflection

  8. Calculating critical angle • n1(low index)/n2(high index) = sin CA • Diamond n =2.42 so the critical angle is?

  9. Can prove this with a prism, which separates white light into its constituent wavelengths/colors Dispersion • Index of refraction is not the same for all wavelengths of light • Violet light more refracted than red

  10. Dispersion • The amount of difference between red and violet light bending

  11. Index of refraction • Minerals can have either 1, 2, or 3 indices of refraction

  12. Relief: contrast between mineral index of refraction and its surroundings • Positive if index higher than surrounding • Negative relief if index lower • Epoxy on slides n=1.56

  13. relief • What if indices of refraction exactly match?

  14. - Olivine has high relief - Plag has low relief plag olivine olivine: n=1.64-1.88 plag: n=1.53-1.57 epoxy: n=1.54 Mineral properties: relief • Relief is a measure of the relative difference in n between a mineral grain and its surroundings • Relief is determined visually, in PPL • Relief is used to estimate n

  15. Mineral properties in PPL: relief • Relief is a measure of the relative difference in n between a mineral grain and its surroundings • Relief is determined visually, in PPL • Relief is used to estimate n garnet: n = 1.72-1.89 quartz: n = 1.54-1.55 epoxy: n = 1.54 Quartz has low relief Garnet has high relief

  16. Hi relief (+) Lo relief (+) Hi relief (-) nxtl > nepoxy nxtl = nepoxy nxtl < nepoxy What causes relief? Difference in speed of light (n) in different materials causes refraction of light rays, which can lead to focusing or defocusing of grain edges relative to their surroundings

  17. How to determine relief Positive relief • Becke Line test • Scope on high power, lower the stage, does halo move in or out? Negative relief

  18. Becke line test: positive or negative relief • Becke line test: positive or negative relief • Degree of relief: low, medium, high

  19. Isotropic minerals • Velocity of light same in all directions • Isometric minerals, glass • Ligh vibrates same way going in as going out

  20. west (left) east (right) Only the component of light vibrating in E-W direction can pass through lower polarizer – light intensity decreases How to ID isotropic minerals 1) Light passes through the lower polarizer Unpolarized light Plane polarized light PPL=plane polarized light

  21. north (back) south (front) Black!! 2) Insert the upper polarizer west (left) east (right) Now what happens? What reaches your eye? Why would anyone design a microscope that prevents light from reaching your eye??? XPL=crossed nicols (crossed polars)

  22. Isotropic minerals • Always black in cross polarized light • How to ID? • Relief (degree of, +, -) • Color • Color is observed only in PPL hbl plag • Plagioclase is colorless • Hornblende is pleochroic in olive greens

  23. Mineral properties: color & pleochroism • Color is observed only in PPL • Not an inherent property - changes with light type/intensity • Results from selective absorption of certain l of light • Pleochroism results when different l are absorbed differently by different crystallographic directions - • rotate stage to observe hbl hbl plag plag • Plagioclase is colorless • Hornblende is pleochroic in olive greens Now do question 2

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