A polarizer filter only lets light through if it is going in a specific direction.
When the axes of 2 polarizer filters (the polarizer and the analyzer) are at 90 degrees to each other there is minimal transmittance of light from a source. Any light that does get through must be due to light being bent by something between the 2 filters.
Elements of compensated polarized light microscopy:
(1) light source
(2) polarizer filter
(3) specimen
(4) compensator filter (often red)
(5) analyzer filter (second polarizer)
(6) eye of the beholder
The red compensator is at 45 degrees to both (in between the 90 degrees). This gives the background a reddish color. It has 2 axes - one fast and one slow.
Terms:
(1) birefringence = in uncompensated polarized light the material "lights up" when the 2 polarizer filters are at 90 degrees to each other (maximum background darkness)
(2) negative birefringence with a red compensator = yellow color to the crystal when it is parallel to the slow axis of the red compensator AND blue color when the crystal is parallel to the fast axis of the red compensator
(3) positive birefringence with a red compensator = yellow color to the crystal when it is parallel to the fast axis of the red compensator AND blue color when the crystal is parallel to the fast axis of the red compensator
(4) extinction = orientation of the crystal relative to the light components so that the crystal takes on the background color
Features of uric acid under compensated polarized light:
(1) negatively birefringence
(2) extinction is rapid and complete; it occurs when the crystal is parallel to the polarizer or analyzer axis with the polarizer and analyzer are at 90 degrees to each other
Features of calcium pyrophosphate dihydrate crystals:
(1) positively birefringence
(2) extinction incomplete when polarizer and analyzer are at 90 degrees to each other; occurs at a point 20 degrees from the polarizer-analyzer extinction ("oblique")
