X-ray is the letter X in the NATO phonetic alphabet.

X-rays (German: Röntgenstrahlen) are a form of electromagnetic radiation with a wavelength approximately in the range of 5 pm - 10 nanometers (corresponding to frequencies in the range 30 PHz - 60 EHz).

X-rays with a wavelength longer than 0.1 nm are called soft X-rays. At wavelengths shorter than this, they are called hard X-rays. Hard X-rays overlap the range of long-wavelength (low energy) gamma rays, however the distinction between the two terms depends on the source of the radiation, not its wavelength: X-ray photons are generated by energetic electron processes, gamma rays by transitions within atomic nuclei.

X-rays are primarily used for diagnostic medical imaging and crystallography.


Among the important early researchers in X-rays were Sir William Crookes, Johann Wilhelm Hittorf, Eugene Goldstein, Heinrich Hertz, Philipp Lenard, Hermann von Helmholtz, Thomas Edison, Nikola Tesla, Charles Barkla, and Wilhelm Conrad Röntgen.

Physicist Johann Hittorf observed tubes with energy rays extending from a negative electrode. These rays produced a fluorescence when they hit the glass walls of the tubes. In 1876 the effect was named "cathode rays" by Eugene Goldstein. Later, English physicist William Crookes investigated the effects of energy discharges on rare gases. He constructed what is called the Crookes tube. It is a glass vacuum cylinder, containing electrodes for discharges of a high voltage electric current. He found, when he placed unexposed photographic plates near the tube, that some of them were flawed by shadows, though he did not investigate this effect. In 1892, Heinrich Hertz began experimenting and demonstrated that cathode rays could penetrate very thin metal foil (such as aluminum). Philip Lenard, a student of Heinrich Hertz, further researched this effect. He developed a version of the cathode tube and studied the penetration of X-rays through various materials. Philip Lenard, though, did not realize that he was producing X-rays.

In April 1887, Nikola Tesla began to investigate X-rays using his own devices as well as Crookes tubes. Tesla did this by experimenting with high voltages and vacuum tubes. From Nikola Tesla's technical publications, it is indicated that he invented and developed a special single-electrode X-ray tube. Tesla's tubes differ from other X-ray tubes in that they have no target electrode. He stated these facts in his 1897 X-ray lecture before the New York Academy of Sciences. The modern term for this process is the bremsstrahlung process, in which a high-energy secondary X-ray emission is produced when charged particles (such as electrons) pass through matter. By 1892, Tesla performed several such experiments; however, he did not categorize the emissions as what was later called X-rays (generalizing the phenonomena as radiant energy). Tesla did not publicly declare his findings nor did he make them widely known. His subsequent X-ray experimentation by vacuum high field emissions led him to alert the scientific community to the biological hazards associated with X-ray exposure.

Hermann von Helmholtz formulated mathematical equations for X-rays. He postulated a dispersion theory before Roentgen made his discovery and announcement. It was formed on the basis of the electromagnetic theory of light (Wiedmann's Annalen, Vol. XLVIII); however, he did not work with actual X-rays.

On November 8 1895, Wilhelm Röntgen, a German scientist, began observing and further documenting X-rays while experimenting with vacuum tubes. Röntgen, on December 28, 1895, wrote a preliminary report "On a new kind of ray: A preliminary communication". He submitted it to the Würzburg's Physical-Medical Society journal. This was the first formal and public recognition of the categorization of X-rays. Röntgen referred to the radiation as "X", to indicate that it was an unknown type of radiation. The name stuck, although (over Röntgen's great objections), many of his colleagues suggested calling them Röntgen rays. They are still referred to as Röntgen rays in some countries. Roentgen received the first Nobel Prize in Physics for his discovery.

In 1895, Thomas Edison investigated materials' ability to fluoresce when exposed to X-rays. He found that calcium tungstate was the most effective substance. Around March 1896, the fluoroscope he developed became the standard for medical X-ray examinations. Nevertheless, Edison dropped x-ray research around 1903 after the death of Clarence Madison Dally, one of his glassblowers. Dally had a habit of testing X-ray tubes on his hands, and acquired a cancer in them so tenacious that both arms were amputated in a futile attempt to save his life.[1]

In 1906, physicist Charles Barkla discovered that X-rays could be scattered by gases, and that each element had a characteristic X-ray. He won the 1917 Nobel prize for this discovery.

The use of X-rays for medical purposes was pioneered by Major John Hall-Edwards in Birmingham, England. In 1908, he had to have his left arm amputated owing to the spread of X-ray dermatitis.

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