x-ray x-ray an x-ray, or, much less commonly, x-radiation, is a penetrating form of high-energy electromagnetic radiation. most x-rays have a wavelength ranging from 10 picometers to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz (3×1016 hz to 3×1019 hz) and energies in the range 145 ev to 124 kev. x-ray wavelengths are shorter than those of uv rays and typically longer than those of gamma rays. in many languages, x-radiation is referred to as röntgen radiation, after the german scientist wilhelm conrad röntgen, who discovered it on november 8, 1895. he named it x-radiation to signify an unknown type of radiation. spellings of x-ray(s) in english include the variants x-ray(s), xray(s), and x ray(s). this article is about the nature, production, and uses of the radiation. for the method of imaging, see radiography. for the medical specialty, see radiology. for other uses, see x-ray (disambiguation). not to be confused with x-wave or x-band. natural color x-ray photogram of a wine scene. note the edges of hollow cylinders as compared to the solid candle. x-rays are also used in ways such as checking for broken bones, detecting certain kinds of diseases, "identification of some metals", and ascertaining the locations of weak points in steel. history pre-röntgen observations and research example of a crookes tube, a type of discharge tube that emitted x-rays before their discovery in 1895, x-rays were just a type of unidentified radiation emanating from experimental discharge tubes. they were noticed by scientists investigating cathode rays produced by such tubes, which are energetic electron beams that were first observed in 1869. many of the early crookes tubes (invented around 1875) undoubtedly radiated x-rays, because early researchers noticed effects that were attributable to them, as detailed below. crookes tubes created free electrons by ionization of the residual air in the tube by a high dc voltage of anywhere between a few kilovolts and 100 kv. this voltage accelerated the electrons coming from the cathode to a high enough velocity that they created x-rays when they struck the anode or the glass wall of the tube. the earliest experimenter thought to have (unknowingly) produced x-rays was william morgan. in 1785, he presented a paper to the royal society of london describing the effects of passing electrical currents through a partially evacuated glass tube, producing a glow created by x-rays. this work was further explored by humphry davy and his assistant michael faraday. when stanford university physics professor fernando sanford created his "electric photography", he also unknowingly generated and detected x-rays. from 1886 to 1888, he had studied in the hermann von helmholtz laboratory in berlin, where he became familiar with the cathode rays generated in vacuum tubes when a voltage was applied across separate electrodes, as previously studied by heinrich hertz and philipp lenard. his letter of january 6, 1893 (describing his discovery as "electric photography") to the physical review was duly published and an article entitled without lens or light, photographs taken with plate and object in darkness appeared in the san francisco examiner. starting in 1888, philipp lenard conducted experiments to see whether cathode rays could pass out of the crookes tube into the air. he built a crookes tube with a "window" at the end made of thin aluminium, facing the cathode so the cathode rays would strike it (later called a "lenard tube"). he found that something came through, that would expose photographic plates and cause fluorescence. he measured the penetrating power of these rays through various materials. it has been suggested that at least some of these "lenard rays" were actually x-rays. in 1889, ukrainian-born ivan puluj, a lecturer in experimental physics at the prague polytechnic who since 1877 had been constructing various designs of gas-filled tubes to investigate their properties, published a paper on how sealed photographic plates became dark when exposed to the emanations from the tubes. helmholtz formulated mathematical equations for x-rays. he postulated a dispersion theory before röntgen made his discovery and announcement. he based it on the electromagnetic theory of light. however, he did not work with actual x-rays. in 1894, nikola tesla noticed damaged film in his lab that seemed to be associated with crookes tube experiments and began investigating this invisible, radiant energy. after röntgen identified the x-ray, tesla began making x-ray images of his own using high voltages and tubes of his own design, as well as crookes tubes. discovery by röntgen wilhelm röntgen on november 8, 1895, german physics professor wilhelm röntgen stumbled on x-rays while experimenting with lenard tubes and crookes tubes and began studying them. he wrote an initial report "on a new kind of ray: a preliminary communication" and on december 28, 1895, submitted it to würzburg's physical-medical society journal. this was the first paper written on x-rays. röntgen referred to the radiation as "x", to indicate that it was an unknown type of radiation. some early texts refer to them as chi-rays having interpreted "x" as the uppercase greek letter chi, χ. the name x-rays stuck, although (over röntgen's great objections) many of his colleagues suggested calling them röntgen rays. they are still referred to as such in many languages, including german, hungarian, ukrainian, danish, polish, czech, bulgarian, swedish, finnish, estonian, slovenian, turkish, russian, latvian, lithuanian, albanian, japanese, dutch, georgian, hebrew, and norwegian. röntgen received the first nobel prize in physics for his discovery. 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 (3.3 ft) 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. hand mit ringen (hand with rings): print of wilhelm röntgen's first "medical" x-ray, of his wife's hand, taken on 22 december 1895 and presented to ludwig zehnder of the physik institut, university of freiburg, on 1 january 1896 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. when she saw the picture, she said "i have seen my death." the discovery of x-rays generated significant interest. röntgen's biographer otto glasser estimated that, in 1896 alone, as many as 49 essays and 1044 articles about the new rays were published. this was probably a conservative estimate, if one considers that nearly every paper around the world extensively reported about the new discovery, with a magazine such as science dedicating as many as 23 articles to it in that year alone. sensationalist reactions to the new discovery included publications linking the new kind of rays to occult and paranormal theories, such as telepathy. advances in radiology taking an x-ray image with early crookes tube apparatus, late 1800s. the crookes tube is visible in center. the standing man is viewing his hand with a fluoroscope screen. the seated man is taking a radiograph of his hand by placing it on a photographic plate. no precautions against radiation exposure are taken; its hazards were not known at the time. surgical removal of a bullet whose location was diagnosed with x-rays (see inset) in 1897 röntgen immediately noticed x-rays could have medical applications. along with his 28 december physical-medical society submission, he sent a letter to physicians he knew around europe (january 1, 1896). news (and the creation of "shadowgrams") spread rapidly with scottish electrical engineer alan archibald campbell-swinton being the first after röntgen to create an x-ray (of a hand). through february, there were 46 experimenters taking up the technique in north america alone. the first use of x-rays under clinical conditions was by john hall-edwards in birmingham, england on 11 january 1896, when he radiographed a needle stuck in the hand of an associate. on february 14, 1896, hall-edwards was also the first to use x-rays in a surgical operation. images by james green, from "sciagraphs of british batrachians and reptiles" (1897), featuring (from left) rana esculenta (now pelophylax lessonae), lacerta vivipara (now zootoca vivipara), and lacerta agilis in early 1896, several weeks after röntgen's discovery, ivan romanovich tarkhanov irradiated frogs and insects with x-rays, concluding that the rays "not only photograph, but also affect the living function". at around the same time, the zoological illustrator james green began to use x-rays to examine fragile specimens. george albert boulenger first mentioned this work in a paper he delivered before the zoological society of london in may 1896. the book sciagraphs of british batrachians and reptiles (sciagraph is an obsolete name for an x-ray photograph), by green and james h. gardiner, with a foreword by boulenger, was published in 1897. the first medical x-ray made in the united states was obtained using a discharge tube of pului's design. in january 1896, on reading of röntgen's discovery, frank austin of dartmouth college tested all of the discharge tubes in the physics laboratory and found that only the pului tube produced x-rays. this was a result of pului's inclusion of an oblique "target" of mica, used for holding samples of fluorescent material, within the tube. on 3 february 1896, gilman frost, professor of medicine at the college, and his brother edwin frost, professor of physics, exposed the wrist of eddie mccarthy, whom gilman had treated some weeks earlier for a fracture, to the x-rays and collected the resulting image of the broken bone on gelatin photographic plates obtained from howard langill, a local photographer also interested in röntgen's work. 1896 plaque published in "nouvelle iconographie de la salpetrière", a medical journal. in the left a hand deformity, in the right same hand seen using radiography. the authors named the technique röntgen photography. many experimenters, including röntgen himself in his original experiments, came up with methods to view x-ray images "live" using some form of luminescent screen. röntgen used a screen coated with barium platinocyanide. on february 5, 1896, live imaging devices were developed by both italian scientist enrico salvioni (his "cryptoscope") and professor mcgie of princeton university (his "skiascope"), both using barium platinocyanide. american inventor thomas edison started research soon after röntgen's discovery and investigated materials' ability to fluoresce when exposed to x-rays, finding that calcium tungstate was the most effective substance. in may 1896, he developed the first mass-produced live imaging device, his "vitascope", later called the fluoroscope, which became the standard for medical x-ray examinations. edison dropped x-ray research around 1903, before the death of clarence madison dally, one of his glassblowers. dally had a habit of testing x-ray tubes on his own hands, developing a cancer in them so tenacious that both arms were amputated in a futile attempt to save his life; in 1904, he became the first known death attributed to x-ray exposure. during the time the fluoroscope was being developed, serbian american physicist mihajlo pupin, using a calcium tungstate screen developed by edison, found that using a fluorescent screen decreased the exposure time it took to create an x-ray for medical imaging from an hour to a few minutes. in 1901, u.s. president william mckinley was shot twice in an assassination attempt while attending the pan american exposition in buffalo, new york. while one bullet only grazed his sternum, another had lodged somewhere deep inside his abdomen and could not be found. a worried mckinley aide sent word to inventor thomas edison to rush an x-ray machine to buffalo to find the stray bullet. it arrived but was not used. while the shooting itself had not been lethal, gangrene had developed along the path of the bullet, and mckinley died of septic shock due to bacterial infection six days later. hazards discovered with the widespread experimentation with x‑rays after their discovery in 1895 by scientists, physicians, and inventors came many stories of burns, hair