X-rays are a high-energy penetrating form of radiation. They are electromagnetic waves with wavelengths nm ( ). Like all electromagnetic radiation, X-rays come in discrete packets known as photons.

X-rays were discovered by Röntgen while experimenting with high voltage vacuum tubes. By setting up very high voltages, electrons are accelerated to high speeds before crashing into the positively charged anode, liberating X-rays. Röntgen tentatively named his discovery "X-rays" ("X" standing for unknown), but the name stuck and has been used ever since. X-rays are extremely useful for medical purposes, although they also cause cancer in high doses (due to cell damage) and must be used judiciously and with adequate shielding.

X-rays were discovered being emitted by the coma of comet Hyakutake with 100 times the intensity of the most optimistic prediction (Glantz 1996). No mechanism has been proposed which is capable of explaining the large X-ray flux.

Electromagnetic Radiation, Electron Microprobe, Gamma Radiation, Radiography, Radio Wave, Röntgen, X-Ray Absorption Edge Spectroscopy, X-Ray Absorption Fine Structure Spectroscopy X-Ray Fluorescence Spectroscopy, X-Ray Line Spectrum, X-Ray Powder Diffraction, X-Ray Transmittance Spectroscopy