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
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
THE GOLDEN DECADE : 1895-1905
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
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
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
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
The Nature of
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
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
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
Maurice Hugh Frederick Wilkins Great Britain in
Medicine for the structure of DNA.
1979 Allan McLeod Cormack USA in Medicine for computed
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
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
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
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
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
"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".
- 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.
- 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.
- 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.
- 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.
- Dictionary of Scientific Biography - vol. 13 Editor-in-Chief,
Charles Coulston Gillispie, Charles Scribner's Sons, New York
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
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.