A crystal is an array of atoms that forms atomic layers known as atomic planes. When an x-ray is passed through a crystal, the x-ray beam is diffracted according the crystal's atomic structure. Using a technique called x-ray crystallography, one can construct the three dimensional atomic structure based upon the diffraction pattern.

X-ray crystallography has been used to uncover the atomic structure of thousands of macromolecules ranging from vitamins to protein complexes. X-ray crystallography was a critical technique in many discoveries that were honored with the Nobel Prize. Perhaps the most famous structure revealed by x-ray crystallography is the double helical structure of DNA.

Photo credit: Jane M. Sawyer

The structure of a crystal can be experimentally determined by Bragg's equation,

nλ = 2dsinθ,

where λ is the wavelength of x-rays, d is the distance between atomic planes, θ is the angle of reflection (in degrees), and n is a positive integer.