When electromagnetic radiation travels through a medium, its speed is given by the index of refraction of the material. When radiation goes from one medium to another, the change in the index of refraction causes its path to bend - and this forms the basis of classical optics. For X-rays, the index of refraction is defined by Rayleigh scattering.

While physicists have used Rayleigh scattering to focus X-rays, the strength of the effect drops off as the inverse square of the X-ray energy. This means that at high X-ray energies - and on into low gamma-ray energies - the radiation is not bent enough for a lens to work effectively. One way round this is to put the radiation through a large number of successive lenses. However, no lens is perfectly transparent and at higher energies the large number of lenses needed would result in practically all of the radiation being absorbed.

According to classical physics and conventional quantum physics, this trend should continue at higher energies. This is what researchers at Ludwig Maximilians University in Munich, Germany, and at the Institut Laue-Langevin in Grenoble, France, set out to measure in silicon. But instead they discovered that the exact opposite occurs - the index of refraction starts to make a comeback at energies greater than about 700 keV. What is more, while the index of refraction is negative for X-rays, it becomes positive for gamma rays.

Possible applications are medical imaging where gamma rays could be used to track lithium in the brains of patients being treated for bipolar disorders. The discovery could also result in a better fundamental understanding of how light interacts with matter.