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Willhelm Conrad Rontgen
X-Rays : The Unknown Glimmer
Dr V B Kamble


Experimentation is ...... the most reliable lever enabling us to extract secrets from nature" -

Wilhelm Conrad Rontgen.

Willhelm Conrad Rontgen (pronounced Ryoentgen) was born at Lennep in Bergischen in the Rhine province of Germany on March 27, 1845. He was the only son of Friedrich Conrad Rontgen, a cloth manufacturer and merchant of Lennep. His mother, Charlotte Coustanze Frowein was born in Holland, although her family came originally from Lennep. When Wilhelm was three, they moved to Holland. Here Wilhelm attended a private boarding school, the Institute of Martinus Herman Van Doorn. He was not a particularly studious boy, but preferred to remain out-of-doors and use his hands. He entered the Utrecht Technical school in 1862 at the age of sixteen. As the story goes, he was expelled from the school for refusing to identify a classmate who had drawn a caricature of one of the teachers! As a result, continuity of his formal progress towards university education was broken, and he was never accepted as a regular student by the University of Utrecht. After two and a half years at the Technical School, and nine months of philosophy classes of the university, he passed an examination to enter the Polytechnic at Zurich, as a student of mechanical engineering.

Apparently, Rontgen was extremely happy in Switzerland, both in his work and in his social life. He received his diploma as a mechanical engineer in 1868 and the degree of Doctor of Philosophy a year later. These qualifications won him the assistantship with the Professor of Physics, August Kundt, whose friendship and support greatly shaped Röntgen's career. It was in Zurich, that Rontgen met his future wife, Anna Bertha Ludwig. In 1871, Rontgen accompanied Kundt to the University of Worzburg. In 1872, he married Bertha (Unfortunately, they had no children, but, they adopted Bertha's niece in 1887).

It was in Worzburg that Roöntgen's academic career truly took off. This was despite the fact that he was refused any academic position due to the lack of formal educational requirements. Shortly after his marrieage, he moved to Strasbourg with Kundt, where he became a tutor at the Agricultural Academy of Hohenheim in 1875, he returned to Strasburg to teach theoretical physics. The series of papers he produced during the period 1876-1878 won him the chair of physics at the University of Giessen. During the period 1879-1888 he worked at Giessen, until the Royal University of Worzburg offered him the joint posts of Professor of Physics and Director of the Physical Institute.

In 1894, he became the rector of the University of Worzburg. In 1895, Rontgen made his momentous discovery of X-rays, which brought him international fame. He was made an honorary doctor of medicine of Worzburg in 1896, and a corresponding member of the Berlin and Munich academes. On November 30, 1896, the Royal Society of London jointly awarded to Rontgen and Philipp Lenard (1862-1947)- about whom we shall read later - the Rumford medal. Columbia University awarded Barnard medal in 1900. His statue was elected on the Potsdam Bridge at Berlin, and he was awarded the first Nobel Prize in Physics in 1901. He gave his prize money to further scientific studies at the University of Worzburg.

In the year 1900, Rontgen moved from Worzburg to the chair of physics and the directorship of the Physical Institute at Munich at the request of the Bavarian Government, where he stayed until 1920.

November 08, 1895

On that Friday afternoon, November 08, 1895 the professor of physics and recently elected rector of the Julius Maximilian University of Worzburg, Germany, was unusually late for dinner. And when he did arrive at the family living quarters above his laboratory in the Physical Institute, he did not speak, ate little and then left abruptly to return to the experiments that had so disturbed him that afternoon.

Only hours before, working in his laboratory at the University of Worzburg in Bavaria, Rontgen was suddenly distracted by a mysterious glimmer in one far-off corner of the room. He took a closer look. The strange gleam came from a piece of paper coated with barium platinocyanide, a substance, he knew, that glowed with an eerie luminescence when exposed to cathode rays. But this time, there could be no rays to reflect: the cathode ray tube he had been working with was covered with a heavy piece of cardboard, and, anyway it was clear across the room! Yet when he turned the cathode ray tube off, the paper stopped glowing. When he turned the tube back on, the light shone eerily again. He put his hand between the cathode ray tube and the coated paper. His hand cast a shadow in the light, and he could see the bones in his hand! He took thecoated paper with him to another room, shut the door and pulled the blinds. It still glowed when the cathode ray tube was turned on. It stopped glowing when the tube was turned off. The mysterious rays that were causing the glow had actually passed through the wall! Rontgen had discovered a new ray, which he later called "X-ray" meaning "unknown ray" - a name that hasstuck, even though his ray is no longer so mysterious. This marked the beginning of the era of atomic physics and of an undreamed of succession of medical applications. Indeed, this was the beginning of the Emergence of Modern Science (Figure).

Rontgen was looking for the invisible high frequency rays that Hermann von Helmholtz had prediced from the Maxwell's theory of electromagnetic radiation. Years earlier, von Helmholtz had prevailed upon both Heinrich Hertz and Rontgen to test the experimental predictions of James Clerk Maxwell's new theory. In 1887 Hertz, at the University of Bonn, produced electric spark discharges and demonstrated the propagation of electromagnetic waves through space. The next year Rontgen verified that a dielectric moving in an electric field induces a magnetic force that acts on the dielectric. More recently von Helmholtz had predicted the existence of electromagnetic radiations with frequencies much higher than the natural frequencies of inducible dipoles in matter. These radiations would therefore interact minimally with matter and exhibit great penetrating power. Since cathode rays were then thought by the German school of physics to be "ether" phenomena (that is, similar to light rays passing through the hypothetical medium "ether"), it was proposed that these high-frequency radiations might be present in cathode-ray discharges. A paradox arose: If these radiations interacted minimally with matter, that would explain why they had not yet been detected-but how could their existence be verified? This was precisely the type of research problem in which Rontgen excelled: the painstaking measurement of difficult-to-detect electromagnetic phenomena. It may be of interest to note that Rontgen was red-green colour-blind, and colour-defective individuals tend to become extremely discriminating observers, unconsciously compensating for their deficiency by correlating shapes, shades and textures of familiar objects with their true colours (Figure).


During the decade 1895 - 1905, several remarkable discoveries, especially in Physics, took place, say, for example; x-rays in 1895, radioactivity and Zeeman effect in 1896, the electron in 1897, quantum theory in 1900 and explanation of photoelectric effect and relativity in 1905. Individually, each discovery had enormous significance, while collectivity, they heralded what we today call “Modern Physics”. This was also the period that witnessed tremendous advances in the field of biochemistry with discoveries like conversion of sugar into alcohol outside the living cell, location of malaria parasite in the anopheles mosquito, and discovery of human blood types. Further, this period witnessed the first trans-Atlantic telegraphic radio transmission and the existence of ionosphere. Technological advances, among others, included the first successful air-plane, first electric locomotive, first vacuum tube, flat disk form of phonograph, first practical photoelectric cell, to name a few.

Practitioners of Classical Physics

It may be of interest to note that the “practitioners” of classical physics of that period claimed that all the great discoveries had already been made and that physics would be reduced merely to measurements of greater and greater accuracy! As a matter of fact, some discoveries did lie in the next decimal place as revealed by the discovery of argon during very accurate measurements of the constituents of air. Surely, they did not have any idea of the shape of things to come!

A Fascinating Story

No doubt, there is no story more fascinating than an account of the development of science as a whole, especially the physical sciences, in the decade 1895-1905. We find that with a few exceptions, the ideas, concepts and the laws of physics have evolved gradually. Only occasionally, do we find a few outstanding discontinuities. The discovery of photoelectricity, x-rays, and radioactivity represent such discontinuities and hence are correctly designated as “discoveries”.

An Accidental Discovery

The enormous advances around 1895 brought into question or directly contradicted theories that appeared to have been strongly supported by experimental evidence. For example, the experiments of Hertz demonstrated, beyond doubt, the fundamental nature of Maxwell’s electromagnetic theory of light. And yet, by an irony of fate — that makes the story of modern physics full of most interesting and dramatic situations — these very experiments of Hertz brought to light the new phenomenon of the photoelectric effect, which played an important role in establishing the quantum theory.

The early experiments:

Rontgen was studying the discharge of electricity through rarefied gases. A large induction coil was connected to a rather highly evacuated tube, the cathode C being at one end and the anode A at the side. The tube was covered "with a somewhat closely fitting mantle of thin black cardboard". With the apparatus in a completely darkened room, he made the accidental observation that "a paper screen washed with barium-platino-cyanide lights up brilliantly and fluoresces equally well whether the treated side or the other be turned toward the discharge tube". The fluorescence was observable two meters away from the appratus. Rontgen soon convinced himself that the agency which caused the fluorescence originated at that point in the discharge tube where the glass walls were struck by the cathode stream in the tube. Röntgen's early training as an engineer and his years as Kundt's assistant in Worzburg formed his lifelong habit of making his own apparatus (there was no laboratory mechanic at Worzburg!). Indeed, he was a meticulous experimenter. He invariably worked alone in the laboratory, and with nothing to disturb his concentration, he was able to develop acute powers of observation.

Realizing the importance of his discovery, Rontgen at once proceeded to study the properties of these new rays-the unknown nature of which, as stated earlier, indicated by calling them "X-rays". In his first communication he recorded, among others, the following observations:

  • All substances are more or less transparent to X-rays. For example, wood 2 to 3 cm thick is very transparent. Aluminum 15 mm thick "weakens the effect considerably, though it does not entirely destroy the fluorescence". Lead glass is quite opaque, but other glass of the same thickness is much more transparent. "If the hand is held between the discharge tube and the screen the dark shadow of the bones is visible within the slightly dark shadow of the hand".
  • Many other substances besides barium-platino-cyanide fluoresce_calcium compounds, uranium glass, rock salt, etc.
  • Photographic plates and films "show themselves susceptible to X-rays". Hence photography provides a valuable method of studying the effects of X-rays (An X-ray photograph of his wife's hand is shown elsewhere in this article).
  • X-rays are neither reflected nor refracted (so far as Rontgen could discover). Hence, "X-rays cannot be concentrated by lenses."
  • Unlike cathode rays, X-rays are not deflected by a magnetic field. They travel in straight lines, as Rontgen showed by means of "pinhole" photographs.
  • X-rays discharge electrified bodies, whether the electrification is positive or negative.
  • X-rays are generated when the cathode rays of the discharge tube strike any solid body. A heavier element, such as platinum, however is much more efficient as a generator of X-rays than is a lighter element such as aluminum.
Indeed, it is a stirring tribute to Röntgen's masterly thoroughness that most of the basic properties of X-rays were described in the paper in which the discovery was first announced. His discovery excited intense interest and work on X-rays began at once in many laboratories both in America and in Europe. It is worth noting that this early work is beautifully illustrative of the qualitative phase of development of a typical field of physics (Figure).

The Nature of X-rays

Rontgen took the first steps in identifying the nature of X-rays by using a system of slits to show that they travel in straight lines and that they are uncharged because they are not deflected by electric or magnetic fields. The discovery of X-rays aroused the interest of all physicists, and many joined in the investigation of their properties. In 1896, Bertin-Sans showed that the X-rays could be scattered from glass or paraffin plate just the way light waves are scattered by particles. In 1899, Haga and Wind provided the evidence for the diffraction of X-rays by using wedge-shaped slits only a few thousandths of a millimetre wide and observing a slight broadening of the image on a photographic plate. This showed that x-rays are a wave motion phenomenon. They concluded through this experiment the wavelength of x-rays to be of the order of 10-8 cm. In 1906, Charles Glover Barkla (1877-1944) proved that X-rays are transverse waves by showing that they can be polarised by scattering from many materials. Following a suggestion by Max von Laue (1879-1960) in 1912, the father and son team, Sir William Henry Bragg (1862-1942) and Sir William Lawrence Bragg (1890-1971),perfected the technique of measuring wavelength of X-rays using a crystal (say Nacl) as a diffraction grating. Rontgen had observed that the X-rays could not be reflected or refracted, however, the first positive evidence for their refraction and reflection came from the work of Stenstrom by passing a beam of X-rays through a crystal. There is, of course, no longer anything unknown about the nature of X-rays. They are electromagnetic radiation of exactly the same nature as visible light, except that their wavelength is several orders of magnitude shorter. This conclusion follows from the properties of X-rays described above with similar properties for visible light. This, however, was postulated by Sir J.J. Thomson (1856-1940) several years before all these properties were known!

Production and measurement of X-rays:

Indeed, tubes for the production of X-rays were similar to a form suggested by Rontgen. They were essentially cathode-ray-tubes in which a residual gas pressure of the order of 10-3 mm Hg provides, when voltage is applied, a few electrons and positive ions. These positive ions, bombarding the cathode C, release electrons which, hurled against the anode A, give rise to X-rays. A curved cathode converges the electrons into a focal spot on A of desired shape and size. In this type of tube, known as the "gas" tube, the anode current, applied voltage, and gas pressure are more or less interdependent, and it is essential that the gas pressure be maintained at the desired value. Various ingenious devices were introduced for accomplishing this. It would be of interest to note here that only much later, in 1913, however, an important improvement was introduced by William David Coolidge (1873-1975), American physical chemist and inventor. He evacuated the tube to the highest attainable vacuum and incorporated in the cathode a hot spiral filament of tungsten to serve as source of electrons. The filament was heated by an adjustable current from a battery. Thus the current of electrons in the tube could be controlled independently of the applied voltage .

For quantitative measurements, the ionization method was early adopted. The discharging effect of X-rays upon charged bodies was traced to ionization of the molecules of the surrounding gas. The effect was found to increase rapidly with density, and also to depend on the nature of the gas, the following being increasingly active in the order given: H2, CO, air, CO2, ether vapour, and CS2. At first the rate of discharge of an electroscope was used in measuring the intensity of an X-ray beam, but later an "ionization chamber" was introduced. The metalline cylinder C is filled with a heavy gas like argon or methyl bromide. X-rays enter the window for making the gas within the cylinder conducting. Due to the electric field maintained between C and the rod r, the rod acquires a charge at a rate which can be measured by an electrometer, which is a measure of the intensity of the X-ray beam

Why did X-rays play hide and seek with earlier workers

The discovery of cathode rays had followed continued improvements in the art of pumping gases out of closed containers. The first step in the chain of discovery leading to X-rays was the Geissler discharge, the same gas discharge now used for advertising displays, such as the neon signs. In the late 1870s William Crookes (1832-1919),applied high vacuum techniques to the Geissler discharge, thereby discovering the "Crookes dark space". At the low pressures Crookes produced, the Geissler discharge disappeared and as the voltage was increased a new type of visible discharge appeared: a beam moving in straight lines from the cathode.

By increasing the applied voltages (necessary to produce the discharge) Crookes also inadvertently produced the conditions for the generation of X-rays. Only a small fraction (on the order of 10-4) of the energy of Crookes's cathode rays was emitted as X-rays. The remainder was dissipated as heat. Therefore, cathode-ray tube operation was normally limited to gas pressures and voltages (approximately 9 kV) sufficient to produce visible beams, but not so great that the glass faces of the tubes would melt where the rays impinged.

In the 1880s Crookes also developed the prototype of the modern X-ray tube. Using a concave cathode to focus cathode rays to a spot on an iridio-platinum anode, he unknowingly optimized the efficiency for production of X-rays. During this research he was occasionally bothered by unaccountable fogging of unexposed photographic plates that he stored near his equipment. On occasion he even returned the plates to their manufacturer as defective.

In 1888, seven years before Röntgen's discovery, Phillipp Lenard, attempted to observe high-frequency ultraviolet radiations from a cathode-ray tube. He failed. Had he evacuated his tube to Crookes's low pressures, he would have had to apply higher voltages that generated energetic X-rays and would have immediately detected fluorescence of crystals placed just outside the blackened 2.4 millimeter-thick quartz face of his tube. But he produced only soft X-rays, which the quartz absorbed completely. Lenard missed the golden opportunity a second time in 1893, when he served as Hertz's assistant. This time he did produce much lower pressures in his cathode-ray tubes, requiring much higher operating voltages. The much-higher-energy cathode rays that resulted were able to penetrate thin aluminum windows into the outside air, where they produced brilliant fluorescence of a calcium sulfide phosphor. Lenard subsequently used fluorescent screens of pentadecylparatolylketone (what a tongue-twister!) crystals. Lenard saw intense, easily visible fluorescence of the ketone screens and intense blackening of photographic plates, but "only for a distance in air of a few centimeters". He did find occasional unexplained blackening of a photographic plate covered by a sheet of cardboard thick enough to stop his cathode rays, and his extracted rays "still showed electrical discharge effects at much greater distances", say, up to 30 cm in air. It is strange that although these were X-ray effects, Lenard was unprepared to recognize them! It apparently did not occur to him to repeat his experiment of 1888 under his improved high-vacuum conditions. It could have been so because he was concentrating on the study of cathode rays, and hence missed the side-effects. Further, his research was interrupted by the sudden death of Hertz on the first day of 1894 and then assuming Hertz's duties as director of Physical Laboratory at Bonn. He spent the year completing the editing and publication of Hertz's three-volume final specific work. By January 1896, he had finally returned to his cathode rays. He discovered that Rontgen Rays (Rontgen strahlen) rather than Lenard Rays ( Lenard Strahlen) were being announced in the newspapers!

X-rays: The Mechanism of Production

Late in the nineteenth century, a series of experiments revealed that electrons are emitted from a metal surface when light of sufficiently high frequency falls upon it. This is called photoelectric effect and it provides convincing evidence that photons (or quanta of light-the "packets" of energy) of light can transfer energy to electrons. The production of X-rays can be likened to the inverse photoelectric effect, in which part or all of the kinetic energy of a moving electron can be converted into a photon.

The Bremsstrahlung X-rays exhibit a continuous x-ray spertrum and is the result of Maxwell's electromagnetic theory which predicts that an accelerated (or decelerated) electric charge will radiate electromagnetic waves, and a rapidly moving electron suddenly brought to rest is certainly decelerated. This is what happens when the electrons moving in a cathode ray discharge tube are suddenly brought to rest at the glass surfac of the tube.

The characteristic X-rays, on the other hand, are produced when an energetic electron strikes the atom and knocks out an electron from the shell it is moving in and thus creating a "hole". An atom with a missing electron in a particular shell would give up most of its excitation energy in the form of an X-ray photon when an electron from an outer shell drops into this "hole", thus giving off an "X-ray characteristic" of that shell. It is conventional to speak of electrons in complex atoms as occupying different shells denoted by the capital letters K. L, M, N, O, ...... etc. Electrons in the orbital K are closest to the nucleus and hence require maximum energy to dislodge them, the electrons in the L shell are further away from the nucleus and require much less energy to be dislodged, and so on. Schematically, the situation is shown in the so-called energy level diagram below.

Suppose that the energetic electron moving in a cathode ray tube knocks out a K-shell electron and that an electron from an outer shell (L, M, N, O, ...... etc.) drops into the "hole" in the K-shell. The resulting possible characteristic K-series X-ray photons produced are shown by Ka, Kb, ... etc. (obviously, in the diagram, the O-shell electron dropping into K-shell (Ke) would give rise to the X-ray with maximum energy). Also shown are L, M and N series of characteristic X-rays Figure .

Bizarre, Serendipitous, Fortuitous

Rontgen made one of the most bizarre and serendipitous observations in the history of science when he held a small lead disk in front of the brightly glowing green screen. What he saw was not only the anticipated dark circular shadow of the disk but also the shadows of the bones of his own fingers - an apparition so unearthly as to undoubtedly stir in him thoughts of his own mortality. He quickly withdrew his hand and the dark skeletal shadows of his fingers moved slowly across the still brightly glowing screen.

He recounted in his lecture at Worzburg, "I _ still believed that I was the victim of deception when I observed the phenomenon of the ray" - he turned to photographic film for greater objectivity and for permanent records. True to his nature, in the following weeks Rontgen was driven to secrecy and to feverish experimental verification and reverification of X-rays. Shortly before Christmas, he invited his wife, Bertha, into the laboratory and had her place her hand for 15 minutes on a film cassette opposite his cathode-ray tube. Little could he have known that the morbid image of the bones of Bertha's fingers would catapult him to worldwide celebrity. It may be remarked that the glimmer of the platinocynide screen at the far end of the Röntgen's laboratory table was orders of magnitude dimmer than the bright fluorescence produced by the cathode rays in the glass walls of the cathode ray tubes used by the experimenters then. In the few minutes required to adjust the curtains to exclude the outside light, his visual sensitivity must have increased by a factor of 1000. The retina has two types of light sensitive cells - rods and cones, so named for their shapes. The rods are sensitive to low intensities of light and enable the viewers to see objects even in dim light. The cones work when there is suficient light intensity, but can register red, blue and green light and so distinguish between the colours. In the dark, his eyes would have changed to "rod" vision from the "cone" vision, thus becoming more sensitive to the dark. Also made such circumstances, peripheral vision helps one see the dim objects much more easily through the corner of the eye rather than directly looking at it. (This is why it is easier to spot a faint star while looking at it sideways rather than looking directly at it!). May be, this factor also helped Rontgen to see the glimmer. In addition, the blue-green fluorescence emission of the heat treated barium platinocynide crystals of Röntgen's test screen was optimally effective in stimulating the rods of his retina.

Nobel Prizes for Research with X-rays

It all began with Rontgen! X-rays have contributed in the growth of our knowledge and advancing the frontiers of science in many field be it atomic physics, crystallography, medicine, structure of haemoglobin and DNA, Computer Axial Tomography (CAT scan), chemistry of proteins and so on. Till date 13 Nobel prizes have been awarded for research with X-rays! Here is a list.

1901 W.C. Rontgen Germany in Physics for the discovery of X-rays.

1914 Max von Laue Germany in Physics for X-ray diffraction from crystals.

1915 Sir William Henry Bragg Great Britain in Physics for crystal structure derived from X-ray diffraction. Sir William Lawrence Bragg Great Britain in Physics for crystal structure derived from X-ray diffraction.

1917 Charles Glover Barkla Great Britain in Physics for characteristic radiation of elements.

1924 Kai Manne George Siegbahn Sweden in Physics for X-ray spectroscopy.

1927 Arthur Holly Compton USA in Physics for scattering of X-rays by electrons.

1936 Petrus Josephus Wilhelmus Debye the Netherlands in Chemistry for diffraction of X-rays and electrons in gases.

1962 Max Ferdinand Perutz Great Britain in Chemistry for the structure of haemoglobin.

John Cowdery Kendrew Great Britain in Chemistry for the structure of haemoglobin.

1962 Francis Harry Compton Crick Great Britain in Medicine for the structure of DNA

James Dewey Watson USA in Medicine for the structure of DNA.

Maurice Hugh Frederick Wilkins Great Britain in Medicine for the structure of DNA.

1979 Allan McLeod Cormack USA in Medicine for computed axial tomography.

Godfrey Newbold Hounsfield Great Britain in Medicine for computed axial tomography.

1981 Kai M. Siegbahn Sweden in Physics for high resolution electron spectroscopy.

1985 Herbert A. Hauptman USA in Chemistry for direct methods to determine X-ray structures.

Jerome Karle USA in Chemistry for direct methods to determine X-ray structures.

1988 Johann Deisenhofer Germany in Chemistry for the structures of proteins that are crucial to photosynthesis.

Rober Huber Germany in Chemistry for the structures of proteins that are crucial to photosynthesis.

Hartmut Michel Germany in Chemistry for the structures of proteins that are crucial to photosynthesis.

What type of X-rays were observed?

In Röntgen's experiments the X-rays were produced by the cathode rays striking the walls of the discharge tube (Better results may be obtained by allowing the cathode rays to impinge on a piece of metal, called an anticathode, placed in their path; the X-rays are then emitted from the anti-cathode). In general, any stream of fast moving, i.e. high energy, electrons - no matter how they are formed will produce X-rays when they lose energy and are slowed down upon striking a suitable material.

This is due to the emission of radiation due to acceleration or deceleration in the atomic fields - also called bremsstrahlung (German for braking radiation - as if applying brakes to electrons and suddenly slowing them down!).

As a rule, the wave lengths of the radiations emitted from an anticathode cover a considerable range, but if the X-rays are allowed to fall on a given material, most are absorbed leaving only radiations with wave lengths characteristic of the elements present in the material; this was recognized much later by Charles Glover Barkla (1877-1944), in England, in 1911. These "characteristic X-rays", as they are called, can be produced in other ways, e.g., by permitting cathode rays of high velocity to impinge directly on a target (anticathode) made of, or containing, the particular element. The rays fall into several groups (or series) distinguished by the letters K, L, M, N, etc., in order of decreasing hardness, i.e., of decreasing energy and ability to penetrate matter (see box). For a given element, the rays of the K series are the most difficult to produce, i.e. they require electrons (cathode rays) of the highest energy; production of the L, M, etc., series can occur at lower and lower energies. For elements of increasing atomic weight, the characteristic X-rays of each series become more and more difficult to excite.

Of the two types of X-rays _ characteristic rays and bremsstrahlung _ only the latter were detected in Röntgen's experiments. (Characteristic X-rays emitted from the silicon atoms of the cathode ray tube's glass face or from an aluminum window were less than 2 keV in energy and could not penetrate to the outside of the tube). Incidentally, one eV (electron-Volt) is the energy an electron possesses while passing through a potential difference of one volt. KeV stands for kilo electron-volts. Judging from his reported relative transmissions in aluminum, Röntgen's bremsstrahlung distribution had maximum energies of 30 - 50 keV, with peak intensities at around 20 - 30 keV as a result of energy losses by the incident electrons in the thick glass face of the tube. At such energies, X-rays interact with matter predominantly by the photoelectric effect.

How the world came to know about the discovery of x-rays:

It seemed impossible that such easily observable effects had not been seen by Lenard, Crookes, J.J. Thomson (who established that cathode rays were indeed a stream of electrons) or any of the other cathode ray researchers during the previous two decades. How is it then only Rontgen saw it _ the glows and the shadows of the bone! But the fact is no one had reported the amazing penetrating power of these new rays. No one had seen bizarre shadows of bones on a fluorescent screen! And that they were different from cathode rays.

In the weeks following his discovery, Rontgen became uncommunicative and preoccupied, working, eating and even sleeping in the laboratory for days at a time. After Christmas, armed with experiments demonstrating the physical reality and unusual properties of X-rays, with shadow photographs of the bones of his wife Bertha's hand and, more prosaically, of a set of brass weights enclosed in a wooden box, Rontgen composed, with uncharacteristic speed, a summary of his results. On December 28, 1895, he asked his good friend Karl Lehmann, president of the Physical Medical Society at Worzburg, to prevail upon the editors of the Sitzungberichte der Physikalisch Medizinischen Gasellschaft zu Wurzburg to include his handwritten manuscript, "Uber eine neue Art von Strahlen" (on a new type of rays), in its December 1895 proceedings, even though the paper had not been presented at the December meeting and even though the proceedings were already at the printers. It was not possible at that late date to include his revolutionary X-ray shadowgraphs.

In the next three days he hurriedly produced enough copies of the crucial shadowgraphs to distribute them, along with preprints of the paper, to the leading physicists in Germany, England, France and Austria. He mailed the packages himself, on New Year's Day 1896. As he did so, he acknowledged his anxieties and his unseemly haste about achieving priority of discovery, remarking to Bertha, "Nun wird man dem teufel zahlen mussen" _ "Now the devil must be paid". On 5 January 1896 Röntgen's discovery was described on the front page of the Sunday Edition of "Die Presse" in Vienna. Soon after, the discovery of X-rays was reported throughout the world, even before the paper was published in the scientific journal.

X-rays change the face of the society:

People immediately saw the potential for the use of X-rays for medical diagnosis (although, unfortunately, it was not until many years later that they discovered that X-rays could also be dangerous). X rays could pass easily through soft-body tissue, while being largely blocked by bone structures or other more solid materials. So if a photographic plate is placed behind a patient, a photo can be taken showing bones as a white shadow on black. Tooth decay looks gray against the white of the teeth. Metal objects also show up clearly, and within four days after Röntgen's news arrived in America, X-rays were used to locate a bullet lodged in a patient's leg. Just three months after Röntgen's announcement, a boy named Eddie McCarthy in Dartmouth, Maine, became the first person to have a broken bone set using the new way to view bones.

Rontgen had caused a great furor, not entirely positive. In the state of New Jersey, legislators worried that X-rays meant the end of personal privacy (they were particularly concerned about the modesty of young women) and proposed legislation to prevent the use of X-rays in opera glasses _ an unnecessary worry, of course.

But for scientists, Röntgen's X-rays (initially known as Rontgenstrahlen - Rontgen Rays) would become one of the greatest tools in biological research, and their discovery marked the beginning of a second scientific revolution in physics. For his discovery, in 1901 Wilhelm Rontgen became the first person ever to be awarded the Nobel Prize in physics.

From Röntgen's X-rays, two big boulders started rolling. One would begin an avalanche of revolutionary new ideas about the atom, and the other would lead to the discovery of a strange instability in certain elements, a characteristic that would enable us to tap nuclear power. We shall glance at these pages of the Golden Decade (1895-1905) in future articles. At the time Rontgen discovered X-rays, however, the idea of an atomic nucleus did not even exist!

X - rays Over 100 years:

X-rays have played and have been playing a significant role in our lives even since their discovery by Rontgen. They make the unseen visible in our bodies; they make possible testing of a wide variety of materials in a non-destructive way; they make our air travels safe. Further, through X-ray diffraction and spectroscopy, they make it possible to probe the order of matter at the atomic level (remember Bragg's law nl = 2 d sinq?
The discovery of X-ray diffraction in crystals laid the foundation for the field of X-ray crystallography. Early in their history, scattering of X-rays _ Compton Scattering _ revealed the energy and momentum distributions of electrons as well as vividly illustrating Heisenberg's uncertainty principle. From hospitals to airports, in physics and in biology labs, in the fabrication of nanostructures of electronics and machinery, X-rays have come to permeate the modern world.

X-rays have had a profound impact in advancing the field of biology. Through the awesome powers of the recombinant DNA technology and synchrotron radiations, as combined in modern day X-ray crystallography, ever - more - complicated molecules and assemblages are being worked out in atomic detail. The insight into the biological processes derived from there studies are transforming cell and molecular biology.

X-ray sources in space are serving as valuable laboratories for astrophysics, nuclear physics, relativity, plasma physics and cosmology. X-rays from space have revealed new objects and physical processes hidden from the view of the optical telescopes. Today we know of some 100000 celestial sources of X-rays. Neutron stars were detected in the X-ray band five years before the first detection of the cosmic microwave background which were followed by discoveries about the hottest and most violent places in the universe. It may be interesting to note that even the basic energy sources for the X-ray and optical emitters detected are different! Visible universe is dominated by the objects that derive energy from nuclear reactions, whereas most objects detected in the X-ray regime are powered by gravity, magnetic fields or kinetic energy. One of the most significant accomplishments of X-ray astronomy is the experimental confirmation that black holes exist! Further nearly 50 supernova remnants have been detected at X-ray wave lengths in the milky way, and other nearby galaxies. It may be stated that several X-ray observations in space have contributed to the program of X-ray astronomy in the past. Now it is the CHANDRA X-ray observatory advancing the frontiers of this wonderful field.

Pride in One's Profession, Not Professional Conceit

Before we end this article, let us briefly look at the values this great and unassuming person stood for and what he thought of science as a profession.

In all, Rontgen wrote fifty eight papers, some with collaborators. His first work was published in 1870 dealing with the specific heats of gases. A few years later, he wrote a paper on the thermal conductivity of crystals. Among the other problems his studied were the electrical and other characteristics of quartz; the influence of pressure on the refractive indices of various fluids; the modification of the planes of polarised light by electromagnetic influences; the variation in the functions of the temperature and the compressibility of water and other fluids; and the phenomena accompanying the spreading of oil drops and water.

Despite several honours having been showered upon him, Rontgen retained the characteristic of a strikingly modest and reticent man. He was a great mountaineer and more than once got into dangerous situations! Amiaable and courteous, he was always understanding the views and difficulties of others.

While his discovery could have been patented for tremendous personal benefit, Rontgen instead gave it to humankind and sought no recognition for himself. He gave the cash he received through the Nobel Prize over to the University of Worzburg. After his discovery of X-rays, Rontgen turned his attention to other experiments. After his wife's death on October 31, 1919 at the age of 80, Rontgen resigned his position at the University. In the inflation that followed the World Wat I, he found his savings of a lifetime wiped out. Rontgen himself died on February 10, 1923 with only his devoted housekeeper nearby. He had been weakened by the cancer of intenstine which was diagnosed only in its terminal stage.

Röntgen's attitude to his profession is clearly defined in the address he gave in 1894, when he became rector of Worzburg University:
"The University is a nursery of scientific research and mental education, a place for the cultivation of ideals for students as well as for teachers. Her significance as such is much greater than her practical usefulness, and for this reason, one should endeavour, in filling vacant places, to choose men who have distinguished themselves as investigators and promoters of science, and not only as teachers; for every genuine scientist, whatever his line, who takes his task seriously, fundamentally follows purely ideal goals and is an idealist in the best sense of the word. Teachers and students of the University should consider it a great honour to be members of this organisation. Pride in one's profession is demanded, but not professional conceit, snobbery or academic arrogance, all of which grow from false egotism".


  1. Wilhelm Conrad Rontgen and the glimmer of light Howard H. Seliger, Physics Today, November, 1995. A beautiful article. The present article draws heavily on it.
  2. Introduction to Modern Physics F.K. Richtmyer, E.H. Kennard, and T. Lauritsen McGraw Hill Book Co., 1955 A classic reference book for historical development of Modern Physics. References to original research works mentioned in this article could be found in this book.
  3. The History of Science from 1895 to 1945 Ray Spangenburg and Diane K. Moser Universities Press (India) Ltd., 1999 A five-volume series with short biographical sketeches of scientists and their work. Highly readable.
  4. Sourcebook on Atomic Energy, Samuel Glasstone, D. Van Nostrand and Co. 1969 Yet another classic. Discusses historical aspects and development of ideas and technology in a lucid style.
  5. Dictionary of Scientific Biography - vol. 13 Editor-in-Chief, Charles Coulston Gillispie, Charles Scribner's Sons, New York 1975

X-rays: Glossary

Important terms used in connection with X-rays are given below. The terms given do not necessarily appear in the present article.

Black holes : A region of space-time from which nothing, not even light can escape, because gravity is so strong. However, as a result of the effects of General Relativity and quantum mechanics, they must radiate like hot bodies emitting a lot of gamma rays (electromagnetic radiation much shorter in wavelength than X-rays) and X-rays, as predicted by Stefphen Hawking.

Bragg's Law : It defines the relationship between wavelengths(l) of X-rays falling on a crystal, spacing of planes of atoms in the crystal (d) and the angle of incidence (q) of X-rays, all of which together contribute to X-rays diffraction effects. Mathematically, the relationship can be written as 2d sin q = nl (where n is an integer). Using this relationship, and a beam of x-ray of known wavelength, the positioning of atoms in a crystal can be determined from the observed pattern of diffracted X-rays.

Bremsstrahlung : Braking (decelerating) radiation, A spectrum of X-rays of different intensities, wavelengths and energies, produced when fast moving electrons are suddenly stopped or slowed down on striking a metal. X-rays produced in this manner are used for medical examination purposes.

CAT scan : Computerised Axial Tomography, a means of imaging internal structures of objects using beams of X-rays that probe different parts of the object from different angles.

Cathode Rays : A beam of electrons emitted from the cathode of a high-vacuum tube.

Cathode Ray Tube : A high vacuum tube in which cathode rays produce a luminous image on a fluorescent screen.

Chandra X-ray Observatory : NASA's advanced X-ray Astrophysics space telescope launched in 1999 for detecting far away sources, producing images with five times greater details than the optical ones.

Characteristic X-ray : X-ray produced when an electron in the higher energy shell in an atom drops into a "hole" in a lower energy shell (created due to the electron in the lower shell being knocked out by another energetic electron in a cathode rays tube).

Compton Scattering : Collision between a photon and an electron. The wavelength of electromagnetic radiation (photon) in the X-ray or Gamma ray region increases when it is scattered by electrons. Part of the photon energy is imparted to the electron.

Crooke's dark space : When pressure in a Cathode-ray-tube is about 1mm of mercury, the cathode glow is detached from the cathode and a dark space appears between the cathode and the cathode glow.

Diffraction of X-rays : Similar to light rays from a single slit or a grating. However, the wavelength of X-rays being very small (of the order of 10-8 cm) one needs to use a crystal in which these are families of parallel planes with characteristic separation defined by arrays of atoms as a grating.

Electron Volt (ev) : Unit of energy widely used in Atomic and Nuclear Physics. It is the energy or work done on an electron (or a charged particle with equal charge) when passing through a potential difference of one volt. 1 ev = 1.602 X 10 joules. 1 KeV = 1000 eV . 1 MeV = 1000,000 eV.

Neutron Stars : A cold star, supported by the exclusion principle repulsion between neutrons. Formed when a star heavier than 1.4 times the mass of the Sun is depleted of its hydrogen fuel and collapses under its own gravity. Fast rotating neutron stars (also known as pulsars) may emit pulses in optical or even in X-ray regions.

Polarisation of X-rays : Being electromagnetic waves and hence transverse, the X-rays can be polarised with their field vibrations taking place in a particular plane.

Radiodiagnosis : The use of X-rays and ionising radiation to identify the cause of physical disorders such as tumours.

Radiography : The technique of producing a photographic image of an optically opaque object by passing a beam of X-rays or Gamma- rays through it, onto a photographic film.

Radiology : The branch of medicine dealing with the use of X-rays, radioisotopes and non-ionising radiation (such as ultrasound) in diagnosis of disease.

Radiotherapy : The use of X-rays or radioisotopes to treat a disease, mainly cancer. Synchrotron Radiation : Intense light or X-rays emitted when electrons move in a circular orbit at relativistic speeds.

Uncertainly Principle : Enunciated by Werner Heisenberg in 1927 which states that position and momentum of a particle both simultaneously cannot be determined accurately. One can accurately measure at a time only the position or momentum of the particle. This principle has become one of the basic tenets of quantum mechanics.

X-ray : 1) Photon produced when are energetic electron loses energy by bremsstrahlung or when an atom in a state of high energy decays to states of lower energy. Their wavelength may range from 10-11 to 10-8 m (i.e. 0.1 to A 100 A). 2) X-ray shadowgraph on a photographic plate when X-rays are made to pass through the object of study.

X-ray Astronomy : The study of X-rays coming mainly from sources lying outside the solar system, like novae and supernovae in the Milky Way Galaxy, and other extragalactic radio sources. Satellites are used for X-ray Astronomy since X-rays are obsorbed by the Earth's atmosphere.

X-ray Crystallography : The study of crystal structures by X-ray diffraction techniques.

X-ray Energy Levels : Energy levels of an atom, usually defined by a set of numbers called the quantum numbers - total quantum number n . n = 1 corresponds to K-shell, n = 2 to L-shell and so on.

X-ray Photoelectric Effect : A high energy photon (corresponding to wavelength in the region 0.1 A to 100 A) knocking out an electron from a shell corresponding to a particular energy level in an atom.

X-ray scattering : Similar to scattering of light rays by particles. Scattering of X-rays can, however, be observed from the parallel planes defined by arrays of atoms in a crystal.

X-ray Spectrometer : An apparatus for measuring wavelength spectrum of the radiation emitted from an X-ray tube using crystal as a diffraction grating, and using Bragg's law.

X-ray Star : A star mainly emitting radiation in the X-ray region (the Sun emits X-rays but radiates mainly in the optical region and hence is not an X-ray star.

X-ray Tube : An electronic device to produce X-rays, essentially a cathode ray tube, but designed to produce X-rays on collision of cathode rays with a metallic target.