Interactive Tutorial about Diffraction Interactive example : Units Basic examples Single atom Pair of atoms Row of atoms Interactive examples 1D crystal builder Polygons of atoms Different atoms Ewald sphere Units Interference Bragg's law Goto Contents The periodicity of the atomic positions in a crystal is in the range of a few to some 2000 Å = Angstroem = 10-10m. 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 X-ray 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. © Th. Proffen and R.B. Neder, 2003