Biomedical Engineering Reference
In-Depth Information
Sample tube containing a
solution of an optically active
solution which causes the
plane of polarized light to
rotate.
Polarizing filter (polarizer)
Non-polarized light
The analyzing filter
(analyzer) which is rotated
until no light passes
through it.
Plane polarized light
Light source
1. 6
Light pathway in a polarized light microscope.
Strain birefringence
- this is due to the presence of internal stresses
within the material.
A research grade polarized light microscope has two polars in the illumi-
nation path. The fi rst polar or polarizer is positioned below the specimen,
closest to the light source, and the second polar or analyzer is positioned
above the specimen. The polarizer transforms the light coming from the illu-
mination source into plane polarized light. If we rotate the two polars until
their vibration directions are perpendicular, a condition known as crossed
polars, the observed fi eld will appear black and this is known as the extinc-
tion position.
If a mixture of particulate materials is placed on the microscope stage and
observed between the crossed polars, some of the particles appear colored,
some appear white and some are invisible against the black background. The
particles that 'disappear' are isotropic and must be in the glassy or amor-
phous state or in the cubic crystal system. The other particles which appear
white or colored are anisotropic (possibly in the crystalline state) and must
have at least two principal RIs. A quick test for the presence of anisotropy
is to observe the material between crossed polars. The optical pathway in
the modern polarized light microscope is shown in Fig. 1.6. A more detailed
explanation of what we observe between crossed polars in the polarized
light microscope follows.
As described earlier in this section, when plane polarized light enters an
optically anisotropic material, the light is resolved into two components
