The Physics of How X-rays were Discovered

There are quite a few videos about how Roentgen discovered the x-ray, but most really skimp on the physics.  What was he doing, why was he doing it, how did he discover the medical x-ray and why did he (correctly) conclude that he had found a high energy invisible light?  Ready for the real story? Let’s go.

It all started on the night of November 8th of 1895.  According to Roentgen, he had been interested in something called cathode rays for a long time, specifically the research of his friend Heinrich Hertz and his assistant Philip Lenard.   Cathode rays were actually discovered 26 years previously, and are invisible rays that emanate from the negative electrode of a vacuum tube and can make glass and phosphors glow.  In 1892, Hertz discovered that cathode rays could travel through thin pieces of aluminum and, two years after that, his assistant, Philip Lenard made a tube with an aluminum “window” that let the cathode rays escape the tube.  Lenard found that his phosphor plate would glow a few inches from the “window”, and he could get electric effects up to 30 cm away[i]!  What Lenard didn’t know was that cathode rays are a beam of electrons (which is why the cathode ray can be moved with a magnet) and bombarding a solid with fast-moving electrons produces x-rays.  In fact, the electrical effects 30 cm away were from x-rays, not the cathode rays.  However, Lenard was using a phosphor made of calcium sulfide, which is made of calcium (atomic number 20 meaning it has 20 protons in the nucleus) and sulfur (atomic number 16), atomic numbers so low that it is nearly transparent to a small number of x-rays, which is why Lenard didn’t notice the x-ray effects on his phosphor screen.  It is also important to note that in order to study the glow in the phosphor from the cathode rays, Lenard studied his screen in a dark room and covered the tube in “light-tight” cardboard so it was “only transparent to cathode rays[ii]” through the aluminum window.  Thus, Lenard made the first cathode ray tube/x-ray machine that you covered and studied from the outside. 

Wilhelm Roentgen
Wilhelm Roentgen

Around this time Roentgen wondered about using a different phosphorescent and see if it could possibly work further from the tube.  For that reason, Roentgen used a barium platinocyanide phosphor that had barium (atomic number 56) and platinum (atomic number 78), which, according to an article published in Physics Today, was, “approximately 100 times more efficient for the detection of x-rays than Lenard’s low-atomic-number screens.[iii]”  This is not to imply that Roentgen thought specifically that heavier elements would display a new type of ray.  Instead, it was more of a case of someone who wanted to try using a different phosphor and see what happened.  In October of 1895, he began to buy and build a variety of different vacuum tubes.  This brings us to the fateful night of November 8th of 1895.  Like Lenard, Roentgen covered a tube with thick cardboard, turned off the lights, and excited his vacuum tube.  As he reached for the screen on the side of his tube he noticed out of the corner of his eye a glimmer of light on the screen, “the effect was one which could only be produced, in ordinary parlance, by the passage of light.  [but] No light could come from the tube, because the shield which covered it was impervious to any light known.[iv]”  The importance of the fact that Roentgen noticed the ray on the side of the tube should not be understated.  Cathode rays (ie. beams of electrons) can make it through thin metals but are easily blocked by insulators like cardboard.  So, Roentgen was expecting to see fluorescence past the aluminum face but he knew that the cardboard blocked any cathode rays as well as all visible and ultraviolet light so he wasn’t expecting anything out the side.  So what was happening?  Roentgen recalled, “It seemed at first a new kind of invisible light.  It was clearly something new, something unrecorded.”

For weeks he secretly studied these waves that he baptized x-rays, (x for unknown) to distinguish them from other rays.[v]  Roentgen then found that all of his tubes would make x-rays, whether the cathode ray hit aluminum or glass.  (By the way, it wasn’t until after Roentgen’s first publication that multiple people, including Roentgen, determined that hitting low mass materials like glass and aluminum with cathode rays did not produce as many x-rays as hitting high mass materials like platinum.  For this reason, within months of Roentgen’s publication, x-ray machines used a focused cathode on a piece of platinum.  However, that is not what Roentgen used in 1895!)

Roentgen Tubes

Roentgen also almost immediately noticed that x-rays have, “a penetrative power to a degree hitherto unknown.[vi]”  Roentgen started to hold up various items in front of the screen from a 1000-page book, a pack of cards, tin foil, to rubber and metal plates.  Roentgen wrote, “we soon discover[ed] that all bodies are transparent to this agent, though in very different degrees.”   He then determined that the density of an object was the decisive feature for its transparency to x-rays.  In fact, we now know that the x-rays that Roentgen produced mostly interacted with the material due to what is called the photoelectric effect which depends on approximately the fourth power of the atomic number of the material it interacts with.  This is why medical x-rays work so well: the calcium in the bones (atomic number 20) gives bones around 10 times more x-ray absorption than the tissue (mix of atomic numbers but mostly hydrogen (1), carbon (6), and oxygen (8)), which makes it easy to make a “shadow image” where the bones are around 10 times darker than the surrounding tissue. 

So how did Roentgen discover the medical x-ray?  Well, one day, while studying the transparency of lead to x-rays he held a small lead disk in front of the plate with his hand in front of a phosphorescent screen.  He then noticed, not only the shadow from the disk but also the bones in his own hand!  He dryly noted in his paper that, “the darker shadow of the bones is seen within the slightly dark shadow-image of the hand itself.”   Roentgen was an amateur photographer and develop the silver salts of the photographic film!  Therefore, to validate his observations, Roentgen noted that, “wherever it has been possible, I have …[photographed] every important observation which I have made with the eye by means of the fluorescent screen.”  It was for that reason on December 22, 1895, Roentgen asked his wife Bertha if she would come to the laboratory and hold her hand on a photographic plate for 15 minutes and made a ghostly photo that showed her bones and the large metal ring on her finger.  This was the world’s first medical x-ray.  When Bertha saw this picture she supposedly exclaimed, “I have seen my own death![vii]”  [Remember, before x-rays the only time you sawbones was in a skeleton]

But what were these new rays?  Roentgen first noticed them on the side of the tube but they were far stronger off of where the cathode ray hit the back of the tube, “If one deviates the cathode rays within the tube by means of a magnet, it is seen that the x-rays proceed from a new point, i.e. again from the end of the cathode rays.[viii]”  He thus correctly concluded that x-rays are produced when x-rays are produced when the cathode ray hits a solid.  Roentgen also found that x-rays (like UV and visible light) are not moved by a magnet so concluded that it was an electromagnetic wave and not a form of cathode rays, which we also think is correct today.  

On December 28th, Roentgen turned in his paper “On a New Kind of Rays” and it was ignored for a mere 8 days when a chance dinner conversation led to the first of thousands of front-page news articles.  How x-rays took over popular culture, the medical community, AND Physics research (not to mention how it destroyed Roentgen’s life) is the next time on the lightning tamers.


[i] Lenard, Phillip, “Cathode Rays in Gases“Annal Dur Physiks und Chem. Band 51, 1894  p. 241

[ii] Lenard, Phillip, “Cathode Rays in Gases“Annal Dur Physiks und Chem. Band 51, 1894  p. 240

[iii] Seliger, Howard “Wilhelm Conrad Röntgen and the Glimmer of Light” Physics Today Nov. 1995 p. 28

[iv] referenced on p. 11 Glasser, Otto Wilhelm Conrad Roentgen and the Early History of the Roentgen Ray 1933

[v] “On a New Type of Ray” Roentegen 1895

[vi] ibid

[vii] p 95 “The Human Body” H Newquist

[viii] “On a New Type of Ray” Roentegen 1895

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