An X-ray is an invisible, highly penetrating electromagnetic radiation of much shorter wavelength (higher frequency) than visible light. The wavelength range for X-rays is from about 10⁻⁸ m to about 10⁻¹¹ m; the corresponding frequency range is from about 3 × 10¹⁶ Hz to about 3 × 10¹⁹ Hz. Photographs made with X-rays are known as radiographs and so radiography has important applications in both medicine and industry.
The German mechanical engineer and physicist Wilhelm Conrad Röntgen discovered X-rays in 1895 while experimenting with discharge tubes. He noticed that a piece of paper covered with barium platinocyanide (a phosphor) fluoresced when a nearby discharge tube was switched on. The discharge tube in question contained an induction coil and the entire apparatus was enclosed in black paper. Röntgen observed that the barium platinocyanide paper would fluoresce when brought within a distance of 2 metres of the discharge tube. He then performed a series of experiments on these “new rays” to differentiate it from already-known phenomena (for example, cathode rays and ultraviolet radiation).
Here is a photograph of Wilhelm Conrad Röntgen: