Letâ€™s try optics again

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```					Let’s try optics again
What we have learned to date
   Refractive Index
   Snell’s Law
   And how it works
   Birefringence or Interference Colors
   Absolute value of the difference in refractive
indices
   Isotropic vs Anisotropic minerals
What we covered in lab
   Color and pleochroism (plane light)
   Anisotropism and interference colors
(crossed polars)
   Extinction angles
   How to recognize twinning
   Relief
Interference colors - review
From Relief to the Becke Line

ngrain >noil

noil = 1.525
ngrain = 1.60

Figures from http://www.gwu.edu/~forchem/BeckeLine/BeckeLinePage.htm
N of the oil greater than that of
the mineral grain

Figures from http://www.gwu.edu/~forchem/BeckeLine/BeckeLinePage.htm
Becke line again
The Becke line results from the concentration of light
either inside or outside of the image of the particle,
depending on whether the mineral grain or the oil
has the larger index of refraction. This refraction of
light at the boundaries creates an optical halo
perceived as the Becke line. This halo is caused by
the concentration of refracted light rays along the
edge of the particle . As you lower the stage or raise
the tube, the Becke line will move toward the region
with higher index of refraction.
Let’s revisit calcite
   What do we know about calcite to date
   a = 4.989, c = 17.062
   Light splits into two rays traveling at two
different velocities
   We can look up that n1=1.486, n2=1.64-1.66
   Thus the birefringence=0.1540-0.1740
   Shows very high interference colors
Slide from Jane Selverstone

fast ray                            Some light is now
able to pass
through the
slow ray                 upper polarizer

mineral
grain
When light gets split:
-velocity changes
-rays get bent (refracted)
-2 new vibration directions
plane polarized
-usually see new colors
light

W          E
lower polarizer
calcite
   In which directions the rays travel in the
mineral.
   How are these directions related to the
crystal lattice?
Some new stuff!!

ε  –epsilon known as the
extraordinary ray
 ω omega known as the ordinary
ray
Where they travel
   Ordinary ray ω vibrates perpendicular to the
c-axis
   Extraordinary ray ε vibrates perpendicular to
the ordinary ray in a plane that contains the
c-axis
   One of these rays travels faster than the
other
What happens in calcite
again?

calcite             calcite
ordinary
ray, w         extraordinary
(stays stationary)       ray, e
(rotates)

Slide from Jane Selverstone
What happens if I orient calcite, so
light passes only along the C-axis?
   I will see only one dot!

   The c-axis in calcite coincides with what we call the
OPTIC axis. Birefringence is zero for light traveling
along the c-axis

   In actual fact all hexagonal and tetragonal minerals
behave the same way in polarized light. There will
be one optic axis—thus known as Uniaxial
Some definitions
   If n of ε is greater than n of ω, then the
mineral is positive

   If n of ε is less than n of ω, then the mineral
is negative

   Thus in a positive mineral, omega is faster
than epsilon (velocity and n are inversely
related)
How could we determine if calcite
were positive or negative?
   What information do we need?

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 views: 2 posted: 9/8/2010 language: English pages: 16