Discovery of X-rays

A print of one of the first X-rays by Wilhelm Röntgen of the left hand of his wife Anna Bertha Ludwig. It was presented to Professor Ludwig Zehnder of the Physik Institut, University of Freiburg, on 1 January 1896. Credit: Wikimedia Commons

The German scientist Wilhelm Conrad Röntgen (1845–1923) discovered X-rays at the Physical Institute of the University of Würzburg on November 8, 1895. He wrote an initial report "Ueber eine neue Art von Strahlen. Vorläufige Mitteilung" ("On a new kind of ray: A preliminary communication") and on December 28, 1895, submitted it to Würzburger Physikal.-medic. Gesellschaft journal. To signify an unknown type of radiation, Röntgen named the discovery X-radiation.

   There are conflicting accounts of his discovery because Röntgen had his lab notes burned after his death, but this is a likely reconstruction by his biographers: Röntgen was investigating cathode rays from a Crookes tube which he had wrapped in black cardboard so that the visible light from the tube would not interfere, using a fluorescent screen painted with barium platinocyanide. He noticed a faint green glow from the screen, about 1 meter away. Röntgen realized some invisible rays coming from the tube were passing through the cardboard to make the screen glow. He found they could also pass through books and papers on his desk. Röntgen threw himself into investigating these unknown rays systematically. Two months after his initial discovery, he published his paper. Röntgen discovered their medical use when he made a picture of his wife's hand on a photographic plate formed due to X-rays. The photograph of his wife's hand was the first photograph of a human body part using X-rays.

   For his discovery Röntgen was awarded the inaugural Nobel Prize in Physics in 1901 “in recognition of the extraordinary services he has rendered by the discovery of the remarkable rays subsequently named after him”. The discovery revolutionized diagnostic medicine, and X-rays became a powerful tool for physical experiments.

Wilhelm Conrad Röntgen in 1900. Credit: Wikimedia Commons

   X-ray is a form of electromagnetic radiation of extremely short wavelength and high frequency, with wavelengths ranging from about 10⁻⁸ to 10⁻¹¹ metre, corresponding frequencies from about 3×10¹⁶ hertz (Hz) to 3×10¹⁹ Hz, and energies in the range 145 eV to 124 keV. X-ray wavelengths and energies lay between that of ultraviolet (longer waves and lower energies) and gamma rays (shorter waves and higher energies). Soft X-rays have lower energy, between 0.1 and 10 keV (10–0.1 nm wavelength), and hard X-rays have higher energies ranging from 10 to 100 keV (0.1–0.01 nm wavelength). 

   X-rays are produced by the acceleration of a charged particle, atomic transitions between discrete energy levels, and the radioactive decay of some atomic nuclei. Each mechanism leads to a characteristic spectrum of X-ray radiation.

 

   X-ray astronomy is an achievement of the space age as the Earth’s atmosphere is completely opaque at photon energies beyond the ultraviolet region. In 1949 the first X-rays from the solar corona were detected by a Geiger counter on a V-2 rocket. In 1962 the first X-ray source outside the solar system — Scorpius X-1 — was discovered.

Image of the Sun in X-rays taken by the XRT instrument on the Hinode satellite.
  Credit: SAO, NASA, JAXA, NAO

 

The Crab Nebula in X-rays. The nebula is powered by a rapidly rotating, highly magnetized neutron star, or pulsar (white dot near the center). Image taken by the Chandra X-ray Observatory. Credit: NASA/CXC/SAO/F.Seward et al

References:

W. C. Röntgen: On a New Kind of Rays. Nature 53, 274–276 (1896). https://doi.org/10.1038/053274b0
The Nobel Prize Outreach: Wilhelm Conrad Röntgen
Encyclopedia Britannica: Wilhelm Conrad Röntgen

 

© 2025, Andrew Mirecki