All that can be affirmed regarding the nature of unpolarized light is that it shows no preferential polarization. When, for instance, unpolarized light enters a quartz crystal in a direction parallel to the axis, the light is decomposed into right- and left-circularly polarized beams of equal intensity. This was demonstrated experimentally by Fresnel. By the use of right- and left-handed quartz crystals in the two paths of an interferometer, interference effects occur whose nature can be definitely predicted on the assumption of the existence of such circularly polarized components. An experiment of this nature has been performed by Langsdorf and DuBridge, but the experiment, in itself, furnishes no information as to the nature of unpolarized light.
RAYMOND T. BIRGE, "On the Nature of Unpolarized Light," J. Opt. Soc. Am. 25, 179-181 (1935)