GAMMA RAYS (usually written 7-Rays) are electromag netic radiations of very high frequency emitted by certain radio active bodies (see RADIOACTIVITY). These radiations possess wave lengths of less than Angstrom unit or of a centi meter. They mark the present upper limit of the spectrum of electromagnetic radiations. (See X-RAYS: Nature of, fig. I.) The various interesting properties possessed by these rays are chiefly due to their extremely high frequency.