Optics
> Polarization > Polarization by Double Refraction
DCS#
6H35.55
CELLOPHANE
APPARATUS
sheet of cellophane
202-21-E6
two polarizers
202-21-E6
overhead projector
201
DESCRIPTION
Fold up a piece of cellophane and
place it
between crossed polarizers on the overhead projector. Different
thicknesses
of cellophane will appear as different colors. Rotate the
cellophane
or the analyzer and the colors change. Rotate analyzer through
90°
to get complementary colors.
NOTES
Cellophane is birefringent and so
perpendicular
components of linearly polarized light travel at different speeds
through
the material, the components get out of phase and the light emerges
elliptically
polarized. The phase difference depends on the thickness of the
material
and the wavelength of the light (since n is wavelength
dependent).
The angle also depends on the thickness of the material, so different
colors
are seen for different numbers of layers of cellophane.
One could also layer cellophane tape
on a
piece of glass.
