Like radio waves and other electromagnetic radiation, X rays have a wide range of wavelengths. Strong, deeply penetrating, and highly destructive rays with short wavelengths are called hard X rays. Those with longer wavelengths and less penetrating power—the type used in medical and dental diagnosis—are known as soft X rays.

X rays can penetrate some substances more easily than others. For example, they penetrate flesh more easily than bone, and bone more easily than lead. Thus they make it possible to see bones within flesh and a bullet embedded in bone. The ability of X rays to penetrate depends not only on their wavelength, but also on the density and thickness of the substance.

X rays affect photographic film in the same way as light rays do. An X-ray photograph is made by passing a beam of X rays through the subject onto photographic film. In a more recent technique, called xeroradiography, an electrostatically charged metal plate is substituted for photographic film. When X rays pass through an object and strike the plate, they discharge it in proportion to the density of the object. To bring out the electrostatic image thus formed, the plate is sprayed with a powder that adheres to the charged area. The powder gathers more thickly in heavily charged areas than in lightly charged spots, producing a detailed picture.

X rays cause certain substances to glow, or fluoresce.