it is a measure of how effective a material is in bending light coming from a vacuum
Index of refraction
light is refracted towards the normal to the
boundary upon entering a material with higher n
and is refracted away from the normal upon
entering a material with lower n
sinθ1/sinθ2 =n2/n1
Snell’s Law
It is the simplest way to measure IR of a mineral, relies on immersion oils of known IR
Immersion method
The degree to which mineral grains stand out from the mounting medium
Relief
Has sharp grain boundaries, show fractures and cleavages well
Relief
mineral and oil/balsam indices differ
by 0.12 or more
High relief
indices are within roughly 0.04 of
each other
Low relief
indices differ by roughly 0.04 –
0.12
Moderate relief
If n_oil = n_mineral, light passes
through the oil-mineral boundary
unrefracted
the grain does not stand out
If noil ≠ nmineral, light passing
through the oil-mineral boundary
is refracted
the grain appears to stand out
Relief of anisotropic / birefringent minerals
in grain mounts or thin sections may ___?
may
change as the microscope stage is rotated
it is the different indices of refraction
associated with different crystallographic
directions
Birefringent
index of mineral > index of
surroundings
Positive relief
index of mineral < index of
surroundings
Negative relief
When mineral grains in a grain mount are slightly out of focus, a band of light called the ____? should be visible along the grain boundaries
Becke Line
