A second atom, at the center of the cell, has been added to preset 07. Watch the diffraction pattern as you add and remove atom B. Click on the right hand lattice button to display the reciprocal lattice. With Atom B in place, there are diffraction peaks at only one half of the reciprocal lattice points. The destructive interference of the scattering from the two atoms has reduced the intensities of the others to zero. Such missing spots are called extinctions.
Turn the reciprocal lattice off and change the position of atom B to B1 = B2 = 0.2. The envelope of the pattern is now cosinusoidal, reminiscent of the two slit optical diffraction pattern. The envelope is the diffraction pattern of the pair of atoms defining the basis of the crystal structure.
Put atom B back at the center of the cell, turn off the reciprocal lattice, and then make the sizes r or the atomic numbers Z (or both) of the two atoms different and interpret the results.
(CAUTION: The display intensities are always normalized to leave the central spot (0 0) at "full white". Some apparent intensity changes can be misleading because they are a result of renormalization driven by interference at the central spot, rather than changes of interference at the observed spot.)