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Seeing the Invisible

Seeing the Invisible

For thousands of years, our knowledge of the world around us was limited to objects we can see with our eyes and more recently with optical microscopes. However, the wish to see what nature denies the human eye has ever been the aspiration of research. The success of an investigation to make the invisible visible depends, among other things, on how light of the correct wavelength and a sufficient intensity can be focused on to a specimen.


During the last four decades, the growth of light sources that are bright and wavelength-selectable (tunable) has markedly expanded the scope of investigation of the structure of matter in the most general sense. Structure can mean the positions of the atoms (atomic structure), the behavior of the electrons around the atomic nuclei (electronic structure), and the spectrum of atomic vibrations in molecules and solids. Magnetic structure is another category in the broad structure spectrum. Composition (what atoms and compounds are present) is yet another kind of generalized structure.

Image 1. Diagram of electron ejection from the innermost shell of an atom via x-ray probe at the Advanced Photon Source (APS). (Credit: Argonne National Laboratory)

The atomic structures of solid materials span the extremes from completely ordered with atoms arrayed around the points of a repeating lattice to completely disordered, akin to the arrangements in liquids but frozen in place. To