Page 161 - Fundamentals of Light Microscopy and Electronic Imaging

144 POLARIZATION MICROSCOPY

pen. Cellulose is clearly birefringent, and the pattern of its action in polarized
light is consistent with cellophane containing arrays of long-chain cellulose
molecules arranged in parallel across the diameter of the sheet. Notice that
the color of the birefringence of a single thickness of cellophane is white.
• Cut the sheet into the shape of a square or rectangle with a scissors or utility
knife so that the arms of the X are perpendicular to the edges of the rectan-
gle. At one corner of the sheet place another mark—this time, a straight line
2 cm long parallel to and 1 cm in from the edge of the sheet. Fold the corner
of the sheet at a 45° angle, folding over the black line, and reexamine the
folded cellophane between the crossed polars. The area of overlap now
appears dark like the background and remains dark as the sheet is rotated. If
not completely dark, slide the folded corner slightly while holding it between
the crossed polars. Extinction occurs because the phase displacement of O
and E rays induced by the first layer is exactly reversed and undone by the
action of the second folded layer, whose orientation, relative to the main
sheet, has been changed by 90°. This is confirmed by looking at the folded
black line, which makes a 90° angle. This is the action of compensation, and
the folded corner is acting as a compensator to nullify its own birefringence.
• Now fold the sheet in half so that opposite edges remain parallel to each
other. Placed between crossed polars at a 45° angle, a brilliant color is seen,
which in most cases will be bright yellow. If the polars are parallel, the inter-
ference color will be blue. The folded sheet now has the properties of a full-
wave plate and is capable of generating interference colors. If the sheet is
rotated by incremental amounts between crossed polars while the analyzer is
rotated back and forth, you will see the various colors of the 1st-order inter-
ference spectrum. Since the retardation introduced by a single thickness of
cellophane is 230 nm, folding the sheet in this way doubles the retardation
to 460 nm, a deep blue color. Remember that for a full-wave plate, rays of a
particular wavelength (here 460 nm) retarded by exactly 1 emerge from the
plate linearly polarized and vibrating in the same plane as the original inci-
dent rays. This component is removed completely at the analyzer. The O- and
E-ray pairs of other wavelengths, being retarded by more or less than 1 wave-
length, emerge from the plate as elliptically polarized light and are partially
passed by the analyzer. Thus, all visible wavelengths excepting the band of
wavelengths near 460 nm are transmitted and are collectively perceived as
the complementary color to blue, which is a yellow interference color.
Inspection of a Michel Lèvy color chart indicates that the yellow color cor-
responds to removal of wavelengths near 460 nm, and one-half this amount
(230 nm) is the amount of retardation for a single thickness of cellophane.
The reason that the color of birefringence of a single sheet of cellophane
looks white is also apparent: The relative phase retardation is too small to
allow removal of a visible wavelength by the analyzer.
• The orientation of the index ellipsoid of the cellophane plate—a yellow-I
plate—must still be determined. For this we require a birefringent reference
object whose index ellipsoid is known. We will use a strip of cellophane tape,