The Polarizing Lens Microscope
Reticle Scale
Objective
4X
10X
40X
Scale Bar (mm)
2.5
1.0
0.25
Lg. Divisions (mm)
0.25
0.1
0.025
Sm. Divisions (mm)
0.05
0.02
0.005
1. You will need to use the reticle scale to determine the thickness of
the following fibers:
a. Dynel:
b. Dog Hair:
c. Human Hair:
2. You will need to measure the diameter of at least 10 particles of
each of the following materials. Include an average and the range for
each slide.
a. Potato starch
c. Rice starch
b. Corn starch
Crystal morphology
Acicular crystal habit
a. Calcite
3. Draw a sketch of the following
minerals and describe the crystal habit
of each one.
c. Biotite
b. Crocidolite
Index of refraction: a measure of how much the speed of light is
reduced inside a medium. For example, typical glass has a refractive
index of 1.5, which means that light travels at 1 / 1.5 = 0.67 times the
speed in air.
Relief: is a qualitative measurement of the degree to which mineral
grains stand out from the mounting medium. If the refractive index of
the mounting medium and the particle is not the same, the light is
refracted on passing from the mounting medium to the grain.
The approximate differences of the index of refraction between the
mounting medium and the particle is high relief when > 0.12,
moderate relief when between 0.12 and 0.04 and low relief when <
0.04
Relief of anisotropic particles may change as the stage is rotated in
plane light.
Plane light
Low relief
High relief
Polarized light
If the index of refraction of the particle is greater than the mounting
medium, the particle has a positive relief and if it is lower the
particle has a negative relief. In both cases, the particle will
appear to stand out.
4. Determine the relative relief (high, moderate, low) of the
following particles. Unless otherwise indicated, the particles are
mounted in a meltmouth media with a refractive index of 1.662.
a. Radiolaria
b. Quartz
c. Corundum
d. Halite
Analyzer (upper polarizer) -- a polarizing
prism located above the microscope stage,
between the objective lens and the eyepiece.
This restricts the transmission of light
vibrating perpendicular to the polarizer. The
analyzer can be slipped in or out of the light
path or rotated for partially crossed
polarized light. Light passing through the
polarizer will not pass through the analyzer
unless the vibration direction of the light is
changed between the two prisms.
Anisotropic minerals can perform this deed.
Polarizer (lower polarizer) -- a polarizing
prism located beneath the microscope stage
(between the light source and the object of
study). This restricts transmission of light to
that vibrating in only one (N-S) direction.
Some microscopes have a different
orientation direction. In effect, it plane
polarizes the incident light beam.
How is light polarized and how does this help us identify
specific minerals?
Becke line: the bright halo near the edge of a transparent particle
immersed in a medium. The halo moves with respect to that edge as
the focal plane of the microscope is changed. The Becke line moves
when the distance between the objective of the microscope and the
preparation is changed. *The Becke line will always move toward the
higher refractive index medium when the distance is increased and
will move toward the lower refractive index medium when the
distance is decreased from the point of critical focus.
Increase the distance between the objective and the sample…
nglass >nmedium
nglass < nmedium
Anisotropic particle: has more than one index of refraction.
Isotropic particle: has only one index of refraction.
Uniaxial particle: has two indices of refraction.
Biaxial particle: has three indices of refraction.
5. Use the Becke line test to determine if the following particles have
and index of refraction greater than, less than or nearly equal to
1.662.
a. Ground glass (isotropic)
b. Quartz (uniaxial)
c. Olivine (biaxial)
d. Human hair (uniaxial)
e. Triacetate (uniaxial)
Pleochroic minerals change color as the stage is rotated when the
sample is observed in plane light. The color changes because the
slow and fast rays (corresponding to two different indices of
refraction) are absorbed differently as they pass through the material
and therefore have different colors.
5. Determine if the following particles are pleochroic (yes or no). If
they are pleochroic, describe the colors when the stage is
rotated.
a. Feldspar
b. Crocidolite
c. Cigarette ash
d. Amphibole/hornblende