This can be
shown experimentally with a crystal plate cut parallel to the axis as follows. A calcite
plate about 0.01 to 0.03 mm thick or a quartz plate 0.2 to 1.0 mm thick is placed in a
beam of plane-polarized light and beyond it a prism spectroscope arrangement as
shown in Fig. 271. With an arc lamp as a source at S, a continuous spectrum will be
formed on the screen MM. If the crystal plate axis is oriented at an angle () = 45°,
this light is polarized as shown schematically in the figure. To test this polarization,
a second nicol is now inserted between C and Sl' When it is crossed with the polarizer,
the intensity will vary sinusoidally through the spectrum, with zeros at those wavelengths
for which the light transmitted by C is plane-polarized with vibrations perpendicular
to the transmission plane of the second nico!. The thicker the plate the
larger the number of dark bands across the spectrum.
With thick plates the combined light of the spectrum will appear white, since