netic properties. Examples are LaSiP 3 ,
K 6 Bi 2 Sn 23 , and Yb 14 MnSb 11 . Compounds of this type are known as
pnictides.
Pockels cell An electro-optical device used to produce population inversion in lasers and a pulse of
radiation. In a Pockels cell, crystals of
ammonium dihydrogenphosphate
are used to convert plane-polarized
light to circularly polarized light
when a potential difference is applied. A Pockels cell can be made
part of a laser cavity. When this happens, a change of polarization occurs
when light is reÛected from a mirror
and the light that is polarized in one
plane is converted into light polarized in the perpendicular plane. The
effect is that the reÛected light does
not stimulate emission. When the
Pockels cell is turned off the polarization does not occur and the energy in
the cavity can be released in the
form of a pulse of stimulated radiation. See also laser.
point group A group of symmetry
elements that leave a point unchanged, used in classifying molecules. Compare space group.
poise A *c.g.s. unit of viscosity
equal to the tangential force in dynes
per square centimetre required to
maintain a difference in velocity of
one centimetre per second between
two parallel planes of a Ûuid separated by one centimetre. 1 poise is
equal to 10
–1 N s m
–2
.
poison 1. Any substance that is injurious to the health of a living organism. 2. A substance that prevents
the activity of a catalyst. 3. A substance that absorbs neutrons in a nuclear reactor and therefore slows
down the reaction. It may be added
intentionally for this purpose or may
be formed as a Üssion product and
need to be periodically removed.
polar compound A compound
that is either ionic (e.g. sodium chloride) or that has molecules with a
large permanent dipole moment (e.g.
water).
polarimeter (polariscope) An instrument used to determine the
angle through which the plane of polarization of plane-polarized light is
rotated on passing through an optically active substance. Essentially, a
polarimeter consists of a light source,
a polarizer (e.g. a sheet of Polaroid)
for producing plane-polarized light, a
transparent cell containing the sample, and an analyser. The analyser is
a polarizing material that can be rotated. Light from the source is planepolarized by the polarizer and passes
through the sample, then through
the analyser into the eye or onto a
light-detector. The angle of polarization is determined by rotating the
analyser until the maximum transmission of light occurs. The angle of
rotation is read off a scale. Simple
portable polarimeters are used for estimating the concentrations of sugar
solutions in confectionary manufacture.
polariscope (polarimeter) A device
used to study optically active substances (see optical activity). The
simplest type of instrument consists
of a light source, collimator, polarizer, and analyser. The specimen
is placed between polarizer and
analyser, so that any rotation of the
plane of polarization of the light can
be assessed by turning the analyser.
polarizability Symbol α. A measure of the response of a molecule to
an external electric Üeld. When a
molecule is placed in an external
electric Üeld, the displacement of
electric charge induces a dipole in
the molecule. If the electric Üeld
strength is denoted E and the electrical dipole moment induced by this
Pockels cell
424
p
www.AzShimi.ir www.AzShimi.com
K 6 Bi 2 Sn 23 , and Yb 14 MnSb 11 . Compounds of this type are known as
pnictides.
Pockels cell An electro-optical device used to produce population inversion in lasers and a pulse of
radiation. In a Pockels cell, crystals of
ammonium dihydrogenphosphate
are used to convert plane-polarized
light to circularly polarized light
when a potential difference is applied. A Pockels cell can be made
part of a laser cavity. When this happens, a change of polarization occurs
when light is reÛected from a mirror
and the light that is polarized in one
plane is converted into light polarized in the perpendicular plane. The
effect is that the reÛected light does
not stimulate emission. When the
Pockels cell is turned off the polarization does not occur and the energy in
the cavity can be released in the
form of a pulse of stimulated radiation. See also laser.
point group A group of symmetry
elements that leave a point unchanged, used in classifying molecules. Compare space group.
poise A *c.g.s. unit of viscosity
equal to the tangential force in dynes
per square centimetre required to
maintain a difference in velocity of
one centimetre per second between
two parallel planes of a Ûuid separated by one centimetre. 1 poise is
equal to 10
–1 N s m
–2
.
poison 1. Any substance that is injurious to the health of a living organism. 2. A substance that prevents
the activity of a catalyst. 3. A substance that absorbs neutrons in a nuclear reactor and therefore slows
down the reaction. It may be added
intentionally for this purpose or may
be formed as a Üssion product and
need to be periodically removed.
polar compound A compound
that is either ionic (e.g. sodium chloride) or that has molecules with a
large permanent dipole moment (e.g.
water).
polarimeter (polariscope) An instrument used to determine the
angle through which the plane of polarization of plane-polarized light is
rotated on passing through an optically active substance. Essentially, a
polarimeter consists of a light source,
a polarizer (e.g. a sheet of Polaroid)
for producing plane-polarized light, a
transparent cell containing the sample, and an analyser. The analyser is
a polarizing material that can be rotated. Light from the source is planepolarized by the polarizer and passes
through the sample, then through
the analyser into the eye or onto a
light-detector. The angle of polarization is determined by rotating the
analyser until the maximum transmission of light occurs. The angle of
rotation is read off a scale. Simple
portable polarimeters are used for estimating the concentrations of sugar
solutions in confectionary manufacture.
polariscope (polarimeter) A device
used to study optically active substances (see optical activity). The
simplest type of instrument consists
of a light source, collimator, polarizer, and analyser. The specimen
is placed between polarizer and
analyser, so that any rotation of the
plane of polarization of the light can
be assessed by turning the analyser.
polarizability Symbol α. A measure of the response of a molecule to
an external electric Üeld. When a
molecule is placed in an external
electric Üeld, the displacement of
electric charge induces a dipole in
the molecule. If the electric Üeld
strength is denoted E and the electrical dipole moment induced by this
Pockels cell
424
p
www.AzShimi.ir www.AzShimi.com