II. Properties of Light and Refraction
A. Wave nature of light: Unpolarized vs plane-polarized light.
B. Index of Refraction n = C / Cm
Where: n is index of refraction
C is speed of light in vacuum.
Cm is speed of light in the medium of interest.Index of Referactive (n) or RI
1. n characteristic for most minerals (Kerr Table 10. 5, pages 193-198).
2. Most minerals have n between 1.50 - 1.80.
3. n can be measured directly by refractometer.
4. Relief determination.
a. Positive ng > nm
b. Zero ng = nm
c. Negative ng < nm
Where: ng = refractive index of grain (mineral )
nm = refractive index of medium (canada balsam or --)
C. Snell's Law
The law is defined as:
n1 sinø1 = n2 sinø2
Where n is the refractive index and ø the corresponding angles as shown.
The refractive index is the ratio of the speed of light in a vacuum to the speed of light in a given medium. So, if the top part of the diagram is air, n1 is the speed of light in air and if the bottom part is glass, n2 is the speed of light in glass,
both relative to the speed of light in a vacuum.
D. Becke Line method to determine relief.
A Becke line is a band or rim of light visible along a grain/crystal boundary in plane-polarized light. It is best seen using the intermediate power lens (or low power in some cases), on the edge of the grain, with the diaphragm stopped down a bit.1. Upper nicol out.
2. Diaphragm medium to small.
3. Focus on grain.
4. Lower stage (increase distance between stage and objective) Becke line moves into substance with greater refractive index.
5. Decrease the distance between stage and objective , the Becke line will move into the substance with low refractive index.
6. When ng = nm, then Becke line will be colored because of dispersion occurring at the grain boundary.
For Exercise:
1. Put only few grains of unknowns on slide.
2. Cover with cover slip.
3. Place ONE drop of oil next cover slip so it seeps under it.
Pick an oil with an refractive index (n) somewhere in the middle initially and go up or down from there.
5. Use Becke line method to determine n of grains to the nearest 0.005[You should be good at seeing Becke line and determining the RI of minerals. This is one of the cheapest and easiest methods of knowing an unknown mineral.]
E. Oblique method of relief determination-see handoutF. Isotropic Medium
1. Light travels with equal ease in all direction within the medium due to uniform optical properties-so only one n.
2. Because light rays are unaffected, when the nicols are crossed, the grains of isotropic minerals will be dark.
3. Isotropic media: minerals of isometric crystal system (garnet, fluorite, etc.).