Page 150 - Fundamentals of Light Microscopy and Electronic Imaging

GENERATION OF ELLIPTICALLY POLARIZED LIGHT BY BIREFRINGENT SPECIMENS 133

majority), the shape is that of an elliptical spiral of varying degree of ellipticity. The com-
ponent of elliptically polarized light that is able to pass through an analyzer varies
depending on the amount of phase shift and is shown in Figure 8-12. The description of
circular or elliptical waveforms is simply a convenient device for visualizing how O- and
E-wave pairs interact with analyzers and optical elements called retardation plates.
Interference between two intersecting waves of light occurs only when their E vec-
tors vibrate in the same plane at their point of intersection. Only when interference
causes a change in the amplitude in the resultant wave can an object be perceived due to
differences in intensity and contrast. The observed intensity from the O and E waves
vibrating in mutually perpendicular planes emergent from a birefringent object is sim-
ply the sum of their individual intensities; no variations in intensity are observed
because interference cannot occur and the object remains invisible. A sheet of cello-
phane held against a single polarizer on a light box is an example of this behavior. Cel-
lophane is a birefringent sheet made up of parallel bundles of cellulose. The optic axis
is parallel to the axis of the bundles and is contained in the plane of the sheet. When
examined in polarized light without an analyzer, elliptically polarized light emerges
from the cellophane, but since there is no interference or change in amplitude, the sheet
remains invisible against the polarizer. However, if the cellophane is examined between
two crossed polars, components of elliptical waves that are parallel to the analyzer are
transmitted and emerge as linearly polarized light. Background rays from the polarizer
are blocked by the analyzer, so the cellophane stands out as a bright object against a dark
background. The sheet appears brightest when its optic axis is oriented at 45° with
respect to the transmission axes of the two crossed polars.