electricÜeld p, the polarizability αis
deÜned by p= αE. To calculate the
polarizability fromÜrst principles it
is necessary to use the quantum me-
chanics of molecules. However, if
regarded as a parameter, the polariz-
ability αprovides a link between
microscopic and macroscopic theo-
ries as in the Clausius–Mossotti
equation and the Lorentz–Lorenz
equation.
polarization 1.See polarization
of light. 2.The formation of prod-
ucts of the chemical reaction in a
voltaic cell in the vicinity of the
electrodes resulting in increased re-
sistance to currentÛow and, fre-
quently, to a reduction in the e.m.f.
of the cell. See also depolarization.
3.The partial separation of electric
charges in an insulator subjected to
an electricÜeld. 4.The separation of
charge in a polar chemical bond.
polarization of lightThe process
of conÜning the vibrations of the
electric vector of light waves to one
direction. In unpolarized light the
electricÜeld vibrates in all directions
perpendicular to the direction of
propagation. After reÛection or trans-
mission through certain substances
(see polaroid) the electricÜeld is
conÜned to one direction and the ra-
diation is said to be plane-polarized
light. The plane of plane-polarized
light can be rotated when it passes
through certain substances (see opti-
cal activity).
In circularly polarized light, the tip
of the electric vector describes a cir-
cular helix about the direction of
propagation with a frequency equal
to the frequency of the light. The
magnitude of the vector remains con-
stant. In elliptically polarized light,
the vector also rotates about the di-
rection of propagation but the ampli-
tude changes; a projection of the
vector on a plane at right angles to
the direction of propagation de-
scribes an ellipse. Circularly and el-
liptically polarized light are produced
using a retardation plate.polarizerA device used to plane-
polarize light (see polarization of
light). *Nicol prisms or *Polaroid
can be used as polarizers. If a polar-
izer is placed in front of a source of
unpolarized light, the transmitted
light is plane-polarized in a speciÜc
direction. As the human eye is un-
able to detect that light is polarized,
it is necessary to use an *analyser to
detect the direction of polarization.
Crossing a polarizer and analyser
causes extinction of the light, i.e. if
the plane of polarization of the polar-
izer and the plane of the analyser are
perpendicular, no light is transmitted
when the polarizer and analyser are
combined. Both a polarizer and an
analyser are components of a *po-
larimeter.
polar moleculeA molecule that
has a dipole moment; i.e. one in
which there is some separation of
charge in the *chemical bonds, so
that one part of the molecule has a
positive charge and the other a nega-
tive charge.
polarography An analytical tech-
nique having an electrochemical
basis. A dropping-mercury electrode
is used as the cathode along with a
large nonpolarizable anode, and a di-
lute solution of the sample. The drop-
ping-mercury electrode consists of a
narrow tube through which mercury
is slowly passed into the solution so
as to form small drops at the end of
the tube, which fall away. In this way
the cathode can have a low surface
area and be kept clean. A variable po-
tential is applied to the cell and a
plot of current against potential (a
polarogram) made. As each chemical
species is reduced at the cathode (in
order of their electrode potentials) a425 polarography
p