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In
physics, Bragg's law is
the result of experiments into the
diffraction of
x-rays off
crystal surfaces at certain
angles, derived by the English
physicists
Sir W.H. Bragg and his son
Sir W.L. Bragg in
1913. Although simple, Bragg's
law confirmed the existence of
real
particles at the atomic scale,
as well as providing a powerful
new tool for studying
crystals in the form of
x-ray diffraction. The Braggs
were awarded the
Nobel Prize in
physics in
1915 for their work in
determining crystal structures
beginning with
NaCl,
ZnS, and diamond.
When X-rays hit an atom,
they make the
electronic cloud move as does
any
electromagnetic wave. The
movement of these charges
re-radiate waves with the same
frequency (blurred slightly
due to a variety of effects); this
phenomenon is known as the
Rayleigh scattering (or
elastic scattering). These
re-emitted X-rays interfere,
giving constructive or destructive
interferences; this is the
diffraction phenomenon.
The interference is
constructive when the phase shift
is proportional to 2π; this
condition can be expressed by
Bragg's law:
where
- n is an integer,
- λ is the
wavelength of
x-rays, and moving
electrons,
protons and
neutrons,
- d is the spacing
between the planes in the atomic
lattice, and
- θ is the angle between the
incident ray and the scattering
planes
According to the 2θ
deviation, the phase shift causes
constructive (left figure) or
destructive (right figure)
interferences
Note that moving particles,
including
electrons,
protons and
neutrons, have an associated
wavelength, as determined by
Louis de Broglie (see
De Broglie wavelength).
