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Bragg's law
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The periodicity of the atomic positions in a crystal is in the
range of a few to some 2000 Å = Angstroem = 10^{10}m.
It is customary to refer to this
base length as the unit cell dimensions, and to measure the
length of space and the position of atoms a multiples of these
base vectors.
Start out with the default value of 1 Å along the x and
y axis. Vary the length of your unit length and
try to answer the questions below:
 How do the values you observe for the atomic positions change
as you modify your unit length ?
 What is special about the values of 5 and 8 Å,
respectively?
Just as in direct space, crystallographers measure the length of
reciprocal space in multiples of the reciprocal lattice cell size
instead of 1/(1 Angstroem) and refer to the vector in reciprocal space as
h. Sometimes you will find, however, measurements reported in units
of Q.
This exercise will help you to familiarize yourself with the
unit lengths h and Q and their respective relationship
to the unit length of 1/(1 Ångstroem). As you start the exercise, you
will see a diffraction pattern as if taken with a Xray camera. Do
not worry about the details of the picture. The dark spots are Bragg
reflections and you will learn more about these later on.
Start out with the default value of 1/(1 Ångstroem) along the x and
y axis.
Vary the length of your unit length and
try to answer the questions below:
 How do the values you observe for the Bragg reflections change
as you modify your unit length ?
 What is a good value for the unit length, i.e. for
which values are the coordinates of the Bragg reflections
integer numbers?
 What is the length of the h unit?
 What is the length of the Q unit?
Once you are done, click
here to verify your answers.
