This technique is a more refined application of the use of reflected light (compared to low-power stereo microscopy). At its most basic, you can simply drag a fiber optic light over to the polarizing microscope, arrange the light carefully to illuminate the field of view, put your hand over the transmitted light source, and look down the microscope. This is very revealing for opaque minerals. This technique may reveal organic matter (brown to black, depending on its alteration state) and reveal fluid inclusions as plans of little white or iridescent specks. A more sophisticated form of reflected light microscopy is applied to the study of rocks that are dominated by opaque minerals. Special microscopes for this technique illuminate the polished specimen with polarized reflected light. Interactions of the polarized light with crystal surfaces can be interpreted with respect to crystallography and mineralogy, just as with transmitted polarized light.
A great deal of useful information can be obtained by applying a reflected light source directly to a thin section on the stage of a transmitted light microscope. Place your hand over the transmitted light source for a reflected light view (A), or, use both light sources simultaneously for combined information. Another alternative (B) is to place a colored card beneath the section in reflected light, to yield dramatic color contrasts within the specimen that may direct your attention to previously unrecognized features.
(A)
(B)