X rays are a form of electromagnetic radiation with wavelengths shorter than ultraviolet radiation and longer than the most energetic gamma radiation in many conventional classifications. They typically occupy approximately 0.01 to 10 nanometres, although the boundary between X rays and gamma rays is defined differently in different contexts and cannot always be assigned solely by wavelength.
X ray photons have higher frequencies and energies than ultraviolet photons, and their energies commonly range from roughly hundreds of electronvolts to hundreds of kiloelectronvolts in practical applications. Like visible light, radio waves and gamma rays, X rays consist of electromagnetic fields and travel through vacuum at the speed of light. Their relatively high photon energies allow them to penetrate materials that strongly absorb visible light. The degree of penetration depends strongly on photon energy, material composition, density, thickness and atomic number. This property makes X rays particularly important for medical imaging, industrial inspection, crystallography, astronomy and scientific research.
X rays can be produced by several physical mechanisms, with two major mechanisms being bremsstrahlung and characteristic X ray emission. Bremsstrahlung, a German term meaning braking radiation, occurs when energetic charged particles, commonly electrons, are rapidly deflected or decelerated by the electric fields of atomic nuclei. The change in electron motion causes electromagnetic radiation to be emitted, producing a broad continuous spectrum whose maximum photon energy is related to the accelerating voltage.
Characteristic X rays arise when an energetic electron removes an electron from an inner atomic shell. An electron from a higher shell can then fall into the vacancy, emitting a photon whose energy corresponds approximately to the difference between the two atomic energy levels. Because atomic energy levels depend on the element, characteristic X ray energies provide information about chemical composition. Modern X ray tubes commonly accelerate electrons through a high voltage towards a metal target, where both bremsstrahlung and characteristic radiation can be generated.
The interaction of X rays with matter depends on photon energy and atomic structure. Important processes include the photoelectric effect, Compton scattering and, at sufficiently high energies, pair production. In the photoelectric effect, an X ray photon transfers its energy to a bound electron and can eject that electron from the atom if the photon energy is sufficient.
Photoelectric absorption is strongly influenced by atomic number and photon energy and is particularly important in diagnostic radiography because tissues with different compositions attenuate X rays differently. In Compton scattering, an X ray photon interacts with an electron, transferring part of its energy and changing direction while the electron recoils. Pair production requires photon energies exceeding 1.022 megaelectronvolts and creates an electron and positron in the field of a nucleus or, under appropriate circumstances, another charged particle field. The relative importance of these processes changes with photon energy and material composition, producing different penetration and attenuation behaviours.
Medical radiography uses differential X ray attenuation to create images of internal structures. An X ray beam passes through the body and is detected on the opposite side by a digital detector. Structures that absorb or scatter more X rays appear differently from structures that allow more radiation to pass through. Bone contains relatively high concentrations of elements and has a density that makes it substantially more attenuating than many soft tissues, so bones are readily distinguished in conventional radiographs.
Computed tomography, or CT, extends the principle by collecting X ray measurements from many directions and using computational reconstruction to produce cross sectional images. Contrast agents containing elements such as iodine can increase the visibility of particular structures because their X ray attenuation differs from surrounding tissues. X ray imaging therefore depends on both physics and reconstruction methods, and image quality involves trade offs among spatial resolution, contrast, noise, exposure and radiation dose.
X rays are ionising radiation because sufficiently energetic X ray photons can remove electrons from atoms and molecules, producing ions. Ionisation can initiate chemical changes and biological damage, including damage to DNA. The biological effect of X ray exposure depends on absorbed dose, dose rate, radiation quality, tissue sensitivity and other factors.
Medical imaging therefore uses the principle of optimisation: the radiation exposure should be kept as low as reasonably achievable while still obtaining the information required for the clinical task. Shielding, beam collimation, appropriate exposure settings, detector efficiency and careful positioning help reduce unnecessary exposure. The fact that X rays can be harmful does not mean that every exposure produces an immediate observable injury. Risk generally depends on accumulated and individual exposure conditions, and medical procedures are designed by considering diagnostic benefit alongside radiation risk.
X rays have many applications outside medicine. Industrial radiography can reveal internal cracks, voids, inclusions, weld defects and structural irregularities without cutting an object apart. Airport and security inspection systems use X ray attenuation and scattering to distinguish materials according to their physical properties. X ray fluorescence, commonly called XRF, measures characteristic X rays emitted by a material after excitation and can be used to determine elemental composition.
X ray diffraction, or XRD, examines the angles and intensities at which X rays are diffracted by ordered atomic structures and is a major method for determining crystal structures, phases and material properties. X ray spectroscopy can investigate electronic states and chemical environments. These applications exploit the relationship between photon energy, atomic structure and material interaction, allowing X rays to function not merely as penetrating illumination but as precise analytical probes.
X ray astronomy reveals objects and processes that emit or strongly modify high energy radiation. Hot gas in galaxy clusters, accretion flows around black holes, supernova remnants, neutron stars, active galactic nuclei and extremely energetic stellar phenomena can produce X rays. Earths atmosphere absorbs much of the incoming cosmic X ray flux, so many astronomical X ray observations require space based telescopes or instruments operating above most of the atmosphere.
X ray detectors must often use specialised mirrors because ordinary optical mirrors do not efficiently reflect high energy photons at normal incidence. Grazing incidence optics can redirect X rays at shallow angles, enabling focusing in suitable energy ranges. Astronomical X ray spectra also provide information about temperature, chemical composition, ionisation state, magnetic activity and high energy particle processes.
X rays and gamma rays are both high energy electromagnetic radiation, and the distinction between them has historically involved different definitions. In one common convention, gamma rays are associated with nuclear or subnuclear processes while X rays are associated with electron transitions or energetic electron interactions, even when their wavelengths overlap. In another classification, wavelength or photon energy is used, making the boundary less absolute.
This illustrates why the origin of radiation can be more useful than a simple wavelength threshold in specialist contexts. X rays should also not be confused with radioactivity itself. Radioactivity describes processes in unstable atomic nuclei, whereas X rays are emitted electromagnetic photons that can be produced by many mechanisms. The discovery of X rays by Wilhelm Conrad Röntgen in 1895 transformed medical and scientific imaging because the radiation could reveal internal structures without conventional dissection. Modern X ray science continues to combine quantum physics, electromagnetism, materials science, medicine, engineering and astronomy.
