Gamma rays are the highest energy form of electromagnetic radiation. Click on image for full size (33K GIF) Windows to the Universe original artwork.

Gamma rays are an extremely high-energy form of electromagnetic (EM) radiation. Gamma ray radiation has a much, much shorter wavelength than visible light, so gamma ray photons have much, much higher energies than photons of light do.

Gamma rays lie at the extreme high-energy end of the electromagnetic spectrum. X-rays, which have slightly lower energies than gamma rays, are the neighbors of gamma rays along the EM spectrum. In fact, the spectral ranges of hard X-rays and gamma rays overlap. Gamma rays have wavelengths of about 100 picometers (100 x 10^-12 meters) or shorter, or energies per photon of at least around 10 keV. This type of electromagnetic wave oscillates with a frequency of 3 exahertz (EHz or 10¹⁸ hertz) or higher.

There is no sharp distinction between the highest energy X-rays and the lowest energy gamma rays. The distinction between X-rays and gamma rays is actually based on the origin of the radiation, not on the frequency or wavelength of the electromagnetic waves. Gamma rays are produced by nuclear transitions, while X-rays are the result of accelerating electrons. Photons with energies between about 10 keV and a few hundred keV can be either hard X-rays or gamma rays.