tities as electrons is described by the
Schrödinger equation. Schrödinger
put forward this formulation of
quantum mechanics in 1926 and in
the same year showed that it was
equivalent to matrix mechanics. Tak-
ing into account the de Broglie
wavelength, Schrödinger postulated
a wave mechanics that bears the
same relation to Newtonian mechan-
ics as physical optics does to geomet-
rical optics.
wave numberSymbol k. The num-
ber of cycles of a wave in unit length.
It is the reciprocal of the wavelength.
wave packet A superposition of
waves with one predominant wave
number k, but with several other
wave numbers near k. Wave packets
are useful for the analysis of scatter-
ing in quantum mechanics. Concen-
trated packets of waves can be used
to describe localized particles of both
matter and photons. The Heisen-
berg uncertainty principle can be
derived from a wave-packet descrip-
tion of entities in quantum mechan-
ics. The motion of a wave packet is in
accord with the motion of the corre-
sponding classical particle, if the po-
tential energy change across the
dimensions of the packet is very
small. This proposition is known as
Ehrenfest’s theorem, named after the
Dutch physicist Paul Ehrenfest
(1880–1933), who proved it in 1927.
wave–particle duality The con-
cept that waves carrying energy may
have a corpuscular aspect and that
particles may have a wave aspect;
which of the two models is the more
appropriate will depend on the prop-
erties the model is seeking to ex-
plain. For example, waves of
electromagnetic radiation need to be
visualized as particles, called *pho-
tons, to explain the photoelectric ef-
fect while electrons need to be
thought of as de Broglie waves in
*electron diffraction.wave vectorA vector kassociated
with a *wave number k. In the case
of free electrons the wave vector kis
related to the momentum pin
*quantum mechanics by p= k,
where is the rationalized *Planck
constant. In the case of *Bloch’s theo-
rem, the wave vector kcan only have
certain values and can be thought of
as a quantum number associated
with the translational symmetry of
the crystal.waxAny of various solid or semi-
solid substances. There are two main
types. Mineral waxes are mixtures of
hydrocarbons with high molecular
weights. ParafÜn wax, obtained from
*petroleum, is an example. Waxes
secreted by plants or animals are
mainly esters of fatty acids and usu-
ally have a protective function.
weak acidAn *acid that is only
partially dissociated in aqueous solu-
tion.weberSymbol Wb. The SI unit of
magneticÛux equal to theÛux that,
linking a circuit of one turn, pro-
duces in it an e.m.f. of one volt as it
is reduced to zero at a uniform rate
in one second. It is named after Wil-
helm Weber (1804–91).Weissenberg technique A tech-
nique used to overcome the problem
of overlapping reÛections in the iden-
tiÜcation of the symmetry and the di-
mensions of a unit cell in *X-ray
crystallography. In this technique, a
screen is placed in front of theÜlm
allowing only one set of reÛections to
be exposed. The Weissenberg tech-
nique produces distorted photo-
graphs, but this can be overcome by
having a coupling between the mo-
tions of the crystal and theÜlm.
Using the precession camera tech-555 Weissenberg technique
w