Polarizing Filters 
Directory
Polarizing Filters cause light to vibrate in one plane. Light traveling along a straight line vibrates in all possible planes. Imagine many radii emanating from a common center. These would represent the many vibrational planes of the light beam. A polarizer cuts out all but one of these.
if two polarizers are oriented at 90o to one another, no light will pass through the second one in the series. Verify this by holding one polarizer while looking at a bright object. Take a second polarizer in your other hand and superimpose it on the first. Turn either one until the light is completely blocked.
If a crystalline or paracrystaile object is placed between crossed polarizers, it will depolarize the light which passes through it. This property is known as birefringence. Consequently, the birefringent material will be visible while all else will remain dark. Cell walls, crystals and some starch grains are birefringent, and become apparent using polarized light. This works with unstained and stained sections.
 Amyloplasts from Cana seen with typical bright-field illumination |
 Amyloplasts from Cana seen with crossed polarizers |