<P> X-radiation (composed of X-rays) is a form of electromagnetic radiation . Most X-rays have a wavelength ranging from 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz (3 × 10 Hz to 3 × 10 Hz) and energies in the range 100 eV to 100 keV . X-ray wavelengths are shorter than those of UV rays and typically longer than those of gamma rays . In many languages, X-radiation is referred to with terms meaning Röntgen radiation, after the German scientist Wilhelm Röntgen, who usually is credited as its discoverer, and who had named it X-radiation to signify an unknown type of radiation . Spelling of X-ray (s) in the English language includes the variants x-ray (s), xray (s), and X ray (s). </P> <P> X-rays with high photon energies (above 5--10 keV, below 0.2--0.1 nm wavelength) are called hard X-rays, while those with lower energy are called soft X-rays . Due to their penetrating ability, hard X-rays are widely used to image the inside of objects, e.g., in medical radiography and airport security . The term X-ray is metonymically used to refer to a radiographic image produced using this method, in addition to the method itself . Since the wavelengths of hard X-rays are similar to the size of atoms they are also useful for determining crystal structures by X-ray crystallography . By contrast, soft X-rays are easily absorbed in air; the attenuation length of 600 eV (~ 2 nm) X-rays in water is less than 1 micrometer . </P>

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