Polarization by Absorption
polarization - dichroicism - Polaroid filters - vector components
What it shows:
Polaroid filters absorb one component of polarization while transmitting the
perpendicular components. The intensity of transmitted light depends on the
relative orientation between the polarization direction of the incoming light
and the polarization axis of the filter.
How it works:
Polaroid filters consist of a parallel array of long-chain molecules whose
electrons can move along the length of the molecules but cannot freely
move across the narrow molecules. If the electric field of the
incident light is parallel to the long molecules, microscopic currents of
electrons are made to flow and the energy of the light is ultimately absorbed
and dissipated as heat (resistance to the current flow). On the other hand,
if the incident light is polarized so its electric field is perpendicular to
the long chains, the electrons can't flow, so there is no net current or
heating or absorption. Thus only light of one polarization (across the
molecules) is transmitted. The Polaroid filters are sandwiched between
two 1/8" thick sheets of Plexiglas, measuring 45 cm square.
1
For ease of use, the polarization axes are marked with yellow tape. A light box
2
makes for a good unpolarized white light source. One filter is placed in front
of the light box and becomes the polarizer; a second filter serves as an
analyzer. The law of extinction, known as
Malus' cos2θ intensity law,
can be demonstrated by rotating the analyzer filter.
Setting it up:
The lecture bench is the best location for viewing -- light extinction is most
complete when the filters are viewed normal to the surface, thus the setup
should be as far away from the front rows as possible so that students sitting
in the side wings won't get too oblique a view. In the very large lecture
halls it might make sense to have the setup on a lecture demo cart which can
be rotated to give the optimum view to the left, center, and right audience.
This of course requires repeating the demonstration three times in succession,
but it doesn't take long and is worth the extra time.
Comments:
This type of polarizing filter was invented by Edwin H. Land in 1928 when he
was a 19-year-old undergraduate here at Harvard. His interest had been
stimulated by reading about some polarizing crystals that had been discovered
in 1852 by William Herepath, an English physician, when his helper
accidentally dropped some iodine in the urine of a dog that had been fed
quinine! Most crystals do not absorb all wavelengths equally and so the
transmitted light is colored. Such crystals are called dichroic
3
because even unpolarized light that passes through them in one direction becomes
a different color than light that passes through in a different direction. Rating **
1
The polarizing material is Type HN32 available from the Polaroid Polarizer
Division, 1 Upland Rd., Norwood MA 02062. It comes in sheets measuring 19"
x 50" x .030" and is a neutral-color linear polarizer of the polyvinyl
alcohol-iodine type. It has a total luminous transmittance of approximately
32% and an extinction transmittance of about .005%. It represents the best
balance between transmission and extinction for general use. If extreme
extinction density is required, one should use Polaroid Type HN22 which has
a total luminous transmittance of approximately 22% and an extinction
transmittance of about 0.0005%. It is slightly green in color. Both types
can be used in the near infra-red region (up to about 850 nm).
2
Logan model 2020 PortaView Light Box slide/transparency viewer; it measures approximately 45x35 cm.
3
The word dichroic has come to mean any material that produces
polarized light by absorption -- so Polaroid is considered dichroic.