Physics of How Wilhelm Roentgen Discovered X-rays

There are quite a lot of videos about how Roentgen discovered the x-ray, but most of them really skimp on the physics. What was he doing? Why was he doing it? How did he discover the medical x-ray? And how did he discover that x-rays are really high-energy invisible light? Ready for the real story? Let's go! According to Roentgen, it all started because he'd been interested in the researches on the cathode ray, especially those of his friend Heinrich Hertz and Hertz's assistant, Philip Leonard. Actually, the cathode ray had been 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, Heinrich Hertz discovered the cathode rays could go through thin pieces of aluminum, and two years after that, his assistant, Philip Leonard, made a tube with an aluminum window that let the cathode rays escape the tube entirely. Leonard found he could get his phosphorous plate to glow a few inches from his tube, and he could even see electrical effects up to 30 centimeters away. What Leonard didn't know was the 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 can produce x-rays. In fact, the electrical effects 30 centimeters away were from x-rays, not the cathode ray. However, Leonard 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 they're nearly transparent to small amounts of x-rays, which is why Leonard didn't notice x-rays with his phosphorous screen. In addition, it is important to note that in order to study his phosphorous screen, Leonard studied his screen in a dark room and covered the tube in light tight cardboard. Thus, Leonard made the first cathode ray tube slash x-ray machine that you covered and studied from the outside. Now, Roentgen had a different phosphor that he was thinking of using with Leonard's tube. Roentgen used a barium plantinoside phosphor that had barium, atomic number 56, and platinum, atomic number 78, which according to an article published in Physics Today was, "...approximately a hundred times more efficient for the detection of x-rays than Leonard's low atomic number screen." This is not to imply that Roentgen thought a heavier element would discover a new type of ray. It's more that he had a new type of phosphor and wanted to see what would happen. In October of 1895, Roentgen began to buy and build a variety of different vacuum tubes. This brings us to the fateful night of November 8th, 1895. Like Leonard, 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. Quote, "...the effect was one which could only be produced 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." The importance that Roentgen noticed the ray on the side of the tube should not be understated. Cathode rays, i.e. beams of electron, can make it through thin pieces of metal 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 of the side. So what was happening? Roentgen recalled, quote, "...it seemed at first a new kind of invisible light. It was clearly something new, something unrecorded. For weeks he secretly studied these rays, which he named X-rays, X for unknown, to distinguish it from any other kind of ray." 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 many people, including Roentgen, found out that hitting low-mass materials like aluminum and glass produces far less X-rays than hitting high-mass materials like platinum. For this reason, within months of Roentgen's publication, X-ray machines were made that used a focused cathode on a piece of platinum. However, that was not what Roentgen used in 1895. Roentgen also knows that X-rays have, quote, "...a penetrative power to a degree hereto so unknown." Roentgen started to hold up various items in front of his screen, from a thousand-page book, a pack of cards, rubber and metal plates. Roentgen wrote, quote, "...we soon discovered that all bodies are transparent to this agent, though in very different degrees." He then determined that the density of the object was the decisive feature for its transparency to X-rays. In fact, we now know that Roentgen's rays interacted with matter due to the photoelectric effect, which depends approximately to the fourth power of their atomic number. This is why medical X-rays work so well. The calcium in the bones, atomic number 20, give bones around 10 times more X-ray absorption than the tissue. A 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 he was studying lead's transparency to X-rays, he held a small lead disk in front of a plate with his hand. 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, quote, "...the darker shadow of the bones is seen within the slightly dark shadow image of the hand itself." Roentgen was an amateur photographer, so he decided to see if the X-rays would develop the film, and he was delighted to find that he did. Therefore, to validate his experiments, Roentgen noted that, quote, "...wherever it has been possible, I've photographed every important observation, which I've made with the eye by means of the fluorescent screen." It was for that reason on December 22nd, 1895, Roentgen asked his wife Bertha if she would come to the laboratory and hold her hand on a photographic plate for 15 minutes. They made a ghostly photo that showed the bones in her hand and her large 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!" Remember, before X-rays, the only time you saw bones was in a skeleton. But what were these new rays? Well, they were noticed at the first on the side of the tube, but they were far brighter on the face, and, quote, "...if one deviates the cathode rays within the tube by means of a magnet, it seemed that the X-rays proceed from a new point, i.e. again from the end of the cathode rays." Roentgen thus correctly concluded that X-rays are created when the cathode ray hits the solid. Roentgen also found that X-rays, like ultraviolet and visible light, are not moved by a magnet. So he concluded that X-rays are an electromagnetic wave and not a form of cathode rays, which is what we think is correct today. On December 8th, 1895, Roentgen turned in his paper on a new kind of ray, and it was ignored for a mere eight days, until suddenly everyone was talking about it, and there was a whole X-ray craze. How X-rays changed our society, our medicine, and our physics, and destroyed Roentgen's life, is next time on the Lightning Tamer. Thanks for watching my video. Please give it a thumbs up if you're interested in more about the different vacuum tubes. I have plenty of videos about that, and the story about how X-ray took over the world is a pretty great one. So check that out. Okay, have a good day. Well one day while he was discover... So how did Roentgen discover the medical X-ray? Well one day while he was this studying, studying! Studying.