X-ray Fundamentals, History, and Applications

Definition and position in electromagnetic spectrum

X-rays are a form of high-energy electromagnetic radiation with wavelengths shorter than ultraviolet light and longer than gamma rays. They typically span wavelengths from about 10 nanometers to 10 picometers, corresponding to photon energies from roughly 100 eV to 100 keV. X-rays were discovered in 1895 by Wilhelm Conrad Röntgen, who named them “X-radiation” to indicate an unknown type of radiation at the time. In the electromagnetic spectrum, X-rays occupy the region between ultraviolet and gamma rays. This position reflects their high photon energies and ability to penetrate many materials, including construction materials and living tissue. Because X-rays are ionizing radiation, exposure can damage DNA and increase risks such as cancer, so their generation and use are tightly controlled. Their spectral range is often discussed in terms of “soft” and “hard” X-rays, which differ by energy and penetration.

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Wavelength, frequency, and photon energy ranges

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