electric Üeld p, the polarizability α is
deÜned by p = αE. To calculate the
polarizability from Ürst principles it
is necessary to use the quantum mechanics of molecules. However, if
regarded as a parameter, the polarizability α provides a link between
microscopic and macroscopic theories as in the *Clausius–Mossotti
equation and the *Lorentz–Lorenz
equation.
polarization 1. See polarization
of light. 2. The formation of products of the chemical reaction in a
*voltaic cell in the vicinity of the
electrodes resulting in increased resistance to current Ûow and, frequently, 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 electric Üeld. 4. The separation of
charge in a polar *chemical bond.
polarization of light The process
of conÜning the vibrations of the
electric vector of light waves to one
direction. In unpolarized light the
electric Üeld vibrates in all directions
perpendicular to the direction of
propagation. After reÛection or transmission through certain substances
(see polaroid) the electric Üeld is
conÜned to one direction and the radiation is said to be plane-polarized
light. The plane of plane-polarized
light can be rotated when it passes
through certain substances (see optical activity).
In circularly polarized light, the tip
of the electric vector describes a circular helix about the direction of
propagation with a frequency equal
to the frequency of the light. The
magnitude of the vector remains constant. In elliptically polarized light,
the vector also rotates about the direction of propagation but the amplitude changes; a projection of the
vector on a plane at right angles to
the direction of propagation describes an ellipse. Circularly and elliptically polarized light are produced
using a retardation plate.
polarizer A device used to planepolarize light (see polarization of
light). *Nicol prisms or *Polaroid
can be used as polarizers. If a polarizer 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 unable 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 polarizer 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 *polarimeter.
polar molecule A 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 negative charge.
polarography An analytical technique having an electrochemical
basis. A dropping-mercury electrode
is used as the cathode along with a
large nonpolarizable anode, and a dilute solution of the sample. The dropping-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 potential 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) a
425
polarography
p
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deÜned by p = αE. To calculate the
polarizability from Ürst principles it
is necessary to use the quantum mechanics of molecules. However, if
regarded as a parameter, the polarizability α provides a link between
microscopic and macroscopic theories as in the *Clausius–Mossotti
equation and the *Lorentz–Lorenz
equation.
polarization 1. See polarization
of light. 2. The formation of products of the chemical reaction in a
*voltaic cell in the vicinity of the
electrodes resulting in increased resistance to current Ûow and, frequently, 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 electric Üeld. 4. The separation of
charge in a polar *chemical bond.
polarization of light The process
of conÜning the vibrations of the
electric vector of light waves to one
direction. In unpolarized light the
electric Üeld vibrates in all directions
perpendicular to the direction of
propagation. After reÛection or transmission through certain substances
(see polaroid) the electric Üeld is
conÜned to one direction and the radiation is said to be plane-polarized
light. The plane of plane-polarized
light can be rotated when it passes
through certain substances (see optical activity).
In circularly polarized light, the tip
of the electric vector describes a circular helix about the direction of
propagation with a frequency equal
to the frequency of the light. The
magnitude of the vector remains constant. In elliptically polarized light,
the vector also rotates about the direction of propagation but the amplitude changes; a projection of the
vector on a plane at right angles to
the direction of propagation describes an ellipse. Circularly and elliptically polarized light are produced
using a retardation plate.
polarizer A device used to planepolarize light (see polarization of
light). *Nicol prisms or *Polaroid
can be used as polarizers. If a polarizer 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 unable 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 polarizer 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 *polarimeter.
polar molecule A 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 negative charge.
polarography An analytical technique having an electrochemical
basis. A dropping-mercury electrode
is used as the cathode along with a
large nonpolarizable anode, and a dilute solution of the sample. The dropping-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 potential 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) a
425
polarography
p
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