When a monochromatic
beam of x-rays
is shone upon a regular crystalline
material then the beam will be scattered from the material at definite angles. This is produced by an interference
effect called diffraction
between the X-rays from different layers
within the crystal.
Crystals consist of regular arrangements of atoms that can be considered to be in layers with specific separations. These layers are separated by distances in the order of Angstroms (0.1nm) apart. This is comparable to the wavelength of x-rays. If x-rays of a known frequency are emitted at a crystalline solid then under particular circumstances of layer separation and angle of incidence to layer, the scatterings from the different layers will constructively interfere (add up), at most angles they will destructively interfere (cancel out).
This effect is characterised by the Bragg Equation, which is the basis of X-ray Crystallography. It was developed by W.H. and W.L. Bragg (Father and Son) who received the 1915 Nobel Prize for Physics for this work.