diffraction pattern
/ dih-FRAK-shun PAT-ern /
Shine a flashlight through a window screen at a wall and you see, instead of a plain bright square, an orderly grid of spots and lines. That arrangement of light and dark is a diffraction pattern. When a crystal scatters X-rays, neutrons, or electrons, it casts exactly this kind of pattern — a constellation of bright spots whose layout is a coded portrait of the atoms inside.
Precisely, a diffraction pattern is the map of where scattered waves reinforce, recorded on a detector or film. Each bright spot marks a direction in which the wavelets from all the atoms added up in step; the dark regions are where they cancelled. The positions of the spots are set by the crystal's repeat spacings — they trace out the reciprocal lattice — while the brightness of each spot is set by what sits inside one repeat unit, through the structure factor. Read together, positions and brightnesses encode the full atomic arrangement.
This matters because the diffraction pattern is the raw data of crystallography: every known crystal structure was decoded from one. The symmetry you see in the pattern mirrors the symmetry of the crystal, so a glance can reveal a great deal. The honest caveat is the phase problem again — the pattern faithfully records how bright each spot is but discards the relative timing of the waves, so turning a pattern back into atomic positions is a real puzzle, not a snapshot you can simply look at.
A single grain of table salt in an X-ray beam casts a square array of spots, because salt's atoms sit on a cubic grid. The fourfold symmetry of the pattern directly mirrors the fourfold symmetry of the crystal — you are quite literally looking at a magnified shadow of the atomic order.
Table salt's cubic order shows up as a square array of diffraction spots.
A diffraction pattern is not a magnified photograph of the atoms — it lives in reciprocal space, so widely spaced spots mean tightly spaced atoms. To get a real-space image of the atoms you must mathematically transform the pattern, not just enlarge it.