Light becomes something quite strange and powerful in the region of the electromagnetic spectrum in which wavelengths are shorter than in the near-UV ultraviolet waveband. This region, shown in Fig. 4.1, includes the extreme ultraviolet, x-ray and gamma-ray wavebands. X rays and gamma rays are electromagnetic waves with such short wavelengths (and correspondingly high energies) that they interact with matter very differently than do the longer wavelengths discussed previously. For the purposes of the following discussion I shall describe x rays and gamma rays in the context of photons, which are particles of light. Light has a dual nature: it can behave as a wave sometimes and a particle at other times. When it is a particle, or photon, it is best described by its energy; that is more appropriate than describing this light as a wave of a particular wavelength.

This method of description reflects the way these high-energy photons interact with matter: when an x ray or a gamma ray does interact with an electron or a nucleus in material, there is a significant amount of energy transferred to a very localized area, like a bullet hitting a metal target. That is why x rays and gamma rays are considered radiation and are dangerous to living beings - the energy transfer often produces permanent chemical changes, which can lead to cellular mutation in living tissue. At the same time, when x rays and gamma rays traverse matter, the probability that they will interact with the matter is fairly low, and if the material is thin or has a low atomic number, then a significant fraction of the light can pass through with little or no loss. The thicker or denser the material, the more reduction in the intensity of a transmitted beam of x rays or gamma rays. This is the principle behind radiography: the film or detector receives more radiation along a path of lower density or thickness through the material being imaged, and less along a thicker or denser path. Thus, the bones of the hand appear darker in a radiograph than the surrounding flesh, since the photographic film is less exposed.

X rays and gamma rays were not initially recognized to be electromagnetic waves at all. Their wave-like properties were hard to detect because their wavelength (~10 nm and smaller) is very short, and it was not until some years after the discovery of x rays that it was conclusively shown that they are electromagnetic waves, producing diffraction patterns on a screen after passing through a narrow metal slit or a crystal.