The reflection of x-rays by crystals.

Discovery of X-ray crystallography. The father and son team of physicists, William Henry Bragg and William Lawrence Bragg, constructed the first X-ray spectrometer using crystals as gratings, using a known wavelength to determine the distances between atomic planes—and thus the structure—of crystalline substances. By the end of 1913 the Braggs reduced the problem of crystal structure analysis to a standard procedure. For further information see the entry at HistoryofInformation.com at this link. The Braggs' paper is available from the Royal Society at this link.

In 1915 the Braggs shared the Nobel Prize in Physics "for their services in the analysis of crystal structure by means of X-rays."