Electromagnetic radiation of very short wavelength. lying within the wavelength interval of 0.1 to 1.5 angstroms (between gamma rays and ultraviolet radiation). X-rays penetrate various thicknesses of all solids, and they act on photographic plates in the same manner as light. Secondary x-rays are produced whenever s-rays are absorbed by a substance. In the case of absorption by a gas, this results in ionization.