Diffraction can be defined as the “spreading of waves around and past obstacles which are comparable in size to their wavelength”. Thus, if the transverse waves below strike a barrier that has an aperture of the correct size (a.), the waves will be propagated on the opposite side as shown (the equally spaced lines and arcs represent the crests of waves):

If the barrier contains a number of such apertures (b.), the multiple waves propagated undergo constructive interference where the wave crests intersect and destructive interference everywhere else.
If the points of intersection are located by “dots”:

it can be seen how this diffraction pattern, perhaps, resembles von Laue's.
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