A polarizing microscope relies on the principles that light has a regular wave motion and the cycle of two rays can be displaced - rotated or put out of phase - with each other.
A normal light microscope is adapted with two polarizing filters in sequence before and after the sample stage in the light path. Therefore, in phase polarizing light passes through the specimen. If there is any regularity of order of molecules around the wavelength of light, this puts the incident beam out of phase relative to light not passing through the same structure. The property of structures to alter the phase of polarized light is termed birefingence. It occurs with various crystalline structures and certain fibres.
The polarizing filter nearer the objective lens and the eye is rotated to reveal the direction of birefringence. Birefringent molecules appear relatively light or dark compared to their surroundings.
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