tracting this has been increased by
the depletion of the ozone layer.
Most UV radiation for practical use
is produced by various types of mercury-vapour lamps. Ordinary glass absorbs UV radiation and therefore
lenses and prisms for use in the UV
are made from quartz.
ultraviolet–visible spectroscopy
(UV–visible spectroscopy) A technique for chemical analysis and the
determination of structure. It is
based on the principle that electronic
transitions in molecules occur in the
visible and ultraviolet regions of the
electromagnetic spectrum, and that a
given transition occurs at a characteristic wavelength.
uncertainty principle (Heisenberg
uncertainty principle; principle of indeterminism) The principle that it is
not possible to know with unlimited
accuracy both the position and momentum of a particle. This principle,
discovered in 1927 by Werner
*Heisenberg, is usually stated in the
form: ΔxΔp x ≥ h/4π, where Δx is the
uncertainty in the x-coordinate of the
particle, Δp x is the uncertainty in
the x-component of the particle’s
momentum, and h is the *Planck
constant. An explanation of the uncertainty is that in order to locate a
particle exactly, an observer must be
able to bounce off it a photon of radiation; this act of location itself alters
the position of the particle in an unpredictable way. To locate the position accurately, photons of short
wavelength would have to be used.
These would have associated large
momenta and cause a large effect on
the position. On the other hand, using
long-wavelength photons would have
less effect on the particle’s position,
but would be less accurate because of
the longer wavelength. The principle
has had a profound effect on scientiÜc thought as it appears to upset the
classical relationship between cause
and effect at the atomic level.
undetermined multipliers See lagrange multipliers.
UNFO Urea nitrate–fuel oil. An explosive based in the fertilizer urea nitrate mixed with fuel oil, similar in
its action to ammonium nitrate–fuel
oil (ANFO). It has been used in a
number of terrorist attacks, most notably a car bombing in the basement
of the World Trade Centre, New
York, on 26 February 1993.
ungerade See gerade.
uniaxial crystal A double-refracting crystal (see double refraction)
having only one optic axis.
unimolecular reaction A chemical reaction or step involving only
one molecule. An example is the decomposition of dinitrogen tetroxide:
N 2 O 4 → 2NO 2
Molecules colliding with other molecules acquire sufÜcient activation
energy to react, and the activated
complex only involves the atoms of a
single molecule.
unit A speciÜed measure of a physical quantity, such as length, mass,
time, etc., speciÜed multiples of
which are used to express magnitudes of that physical quantity. For
scientiÜc purposes previous systems
of units have now been replaced by
*SI units.
unit cell The group of particles
(atoms, ions, or molecules) in a crystal that is repeated in three dimensions in the *crystal lattice. See also
crystal system.
univalent (monovalent) Having a
valency of one.
universal constants See fundamental constants.
universal indicator A mixture of
541
universal indicator
u
www.AzShimi.ir www.AzShimi.com
the depletion of the ozone layer.
Most UV radiation for practical use
is produced by various types of mercury-vapour lamps. Ordinary glass absorbs UV radiation and therefore
lenses and prisms for use in the UV
are made from quartz.
ultraviolet–visible spectroscopy
(UV–visible spectroscopy) A technique for chemical analysis and the
determination of structure. It is
based on the principle that electronic
transitions in molecules occur in the
visible and ultraviolet regions of the
electromagnetic spectrum, and that a
given transition occurs at a characteristic wavelength.
uncertainty principle (Heisenberg
uncertainty principle; principle of indeterminism) The principle that it is
not possible to know with unlimited
accuracy both the position and momentum of a particle. This principle,
discovered in 1927 by Werner
*Heisenberg, is usually stated in the
form: ΔxΔp x ≥ h/4π, where Δx is the
uncertainty in the x-coordinate of the
particle, Δp x is the uncertainty in
the x-component of the particle’s
momentum, and h is the *Planck
constant. An explanation of the uncertainty is that in order to locate a
particle exactly, an observer must be
able to bounce off it a photon of radiation; this act of location itself alters
the position of the particle in an unpredictable way. To locate the position accurately, photons of short
wavelength would have to be used.
These would have associated large
momenta and cause a large effect on
the position. On the other hand, using
long-wavelength photons would have
less effect on the particle’s position,
but would be less accurate because of
the longer wavelength. The principle
has had a profound effect on scientiÜc thought as it appears to upset the
classical relationship between cause
and effect at the atomic level.
undetermined multipliers See lagrange multipliers.
UNFO Urea nitrate–fuel oil. An explosive based in the fertilizer urea nitrate mixed with fuel oil, similar in
its action to ammonium nitrate–fuel
oil (ANFO). It has been used in a
number of terrorist attacks, most notably a car bombing in the basement
of the World Trade Centre, New
York, on 26 February 1993.
ungerade See gerade.
uniaxial crystal A double-refracting crystal (see double refraction)
having only one optic axis.
unimolecular reaction A chemical reaction or step involving only
one molecule. An example is the decomposition of dinitrogen tetroxide:
N 2 O 4 → 2NO 2
Molecules colliding with other molecules acquire sufÜcient activation
energy to react, and the activated
complex only involves the atoms of a
single molecule.
unit A speciÜed measure of a physical quantity, such as length, mass,
time, etc., speciÜed multiples of
which are used to express magnitudes of that physical quantity. For
scientiÜc purposes previous systems
of units have now been replaced by
*SI units.
unit cell The group of particles
(atoms, ions, or molecules) in a crystal that is repeated in three dimensions in the *crystal lattice. See also
crystal system.
univalent (monovalent) Having a
valency of one.
universal constants See fundamental constants.
universal indicator A mixture of
541
universal indicator
u
www.AzShimi.ir www.AzShimi.com