The diffraction of short electromagnetic waves by a crystal.

At the age of 22, Bragg discovered that the regular pattern of dots produced on a photographic plate by an X-ray beam passing through a crystal could be regarded as a reflection of electromagnetic radiation from planes in a crystal that were especially densely studded with atoms. From this work the younger Bragg derived the “Bragg relation” or Bragg's law (nλ = 2d sin O). This relates the wavelength of the X-ray to the angle at which such a reflection could occur. See also: W. H. Bragg, “X-rays and Crystals,” Nature 90 (23 Jan. 1913) 572.