X-ray scattering
/ EKS-ray SKAT-er-ing /
When sunlight passes through a fine net curtain, it spreads into a pattern of bright and dark spots that secretly encodes the spacing of the threads. X-ray scattering reads matter the same way: X-rays, a kind of light with waves far shorter than visible light, wash over the orderly rows of atoms in a material and scatter into a precise pattern that encodes exactly how those atoms are spaced.
X-rays work for this because their wavelength is about the same size as the gaps between atoms — roughly a tenth of a nanometer — which is the condition for the scattered waves to interfere and pile up into sharp spots. The X-rays interact mainly with the electrons surrounding each atom, so a heavy atom with many electrons scatters strongly and a light one weakly. From the angles and brightnesses of the scattered beams, one works backward to reconstruct where the atoms sit and how densely their electrons are spread.
This matters because X-ray scattering is the everyday workhorse for telling what a crystal is made of and how it is built, from minerals to medicines to the structure of proteins. The honest caveat is that it sees electrons, so it is nearly blind to hydrogen and other light atoms that carry few of them, and it generally needs well-ordered material; a truly disordered or constantly shifting sample blurs the sharp spots into faint haze.
In 1952 the famous X-ray scattering image known as 'Photo 51', taken of DNA fibers, showed a cross-shaped pattern of spots that gave away the molecule's double-helix shape (it became famous when published the next year).
'Photo 51' of DNA: the spot pattern of scattered X-rays betrayed the double helix.
Diffraction is one important case of scattering, where ordered atoms produce sharp spots. 'X-ray scattering' is the broader idea — it also covers the diffuse, spread-out signal from liquids, glasses, and electron-density variations, not just neat crystal spots.