Perkin-Elmer MultiScope System, User Reference, Page: 98 / 142

98 . MultiScope System Microscope User's Reference
If a second polarizer is placed in the path of the polarized light two things may
result:
• If the second polarizer is placed in the same direction as the first (as at the
top of Figure 24), the polarized light can pass straight through.
•
However, if the second is placed at a right angle to the first, a situation which
is referred to as
crossed polarizers
, the passage of the polarized light is
blocked, that is,
extinction
occurs (as at the bottom of Figure 24).
The second occurs because the light transmitted by the first polarizer
oscillates in exactly the plane that is blocked by the second polarizer.
Figure 24 Polarizers Parallel (top) and Polarizers Crossed (bottom)
Some materials are anisotropic (or birefringent ): their refractive index depends on
their orientation. These materials can alter the polarization of light passing through
them; this is dependent on the wavelength of the transmitted light.
When you look at an anisotropic sample using polarized light, the change in
polarization caused by the sample means that some light leaks through the second
polarizer. Because the change in polarization is dependent on the wavelength, the
color of the light emerging changes with the distance traveled through the sample
and the amount of birefringence encountered.