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In 1895, a German physicist named Wilhelm Conrad Röntgen was working with a covered cathode ray tube, a glass vacuum-sealed tube that fluoresced when an electron beam passed through it and hit the glass wall at one of the ends.
Discover how radioactivity was discovered, from Röntgen's X-rays to Becquerel's uranium experiments and the Curies' groundbreaking research.
Ask about this video. Answers come from its transcript only — with the timestamp, so you can check them.
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Key Takeaways
- Radioactivity was discovered accidentally through experiments with uranium and photographic plates.
- The phenomenon is a spontaneous emission of radiation from certain elements, independent of external energy like sunlight.
- Radioactivity differs fundamentally from previously known radiation such as X-rays and visible light.
- The discovery opened new scientific fields and led to the isolation of new elements like radium.
- The work of Becquerel and the Curies was foundational and recognized with a Nobel Prize.
What the video covers
- Wilhelm Conrad Röntgen discovered X-rays in 1895, revealing a new form of invisible radiation.
- Antoine Henri Becquerel, inspired by X-rays, experimented with uranium salts and photographic plates to explore phosphorescence and radiation.
- Becquerel discovered that uranium emitted radiation even without exposure to sunlight, leading to the discovery of spontaneous radioactivity.
- He tested various uranium compounds and other materials, concluding that the radiation depended on uranium content, not phosphorescence.
- Becquerel's radiation did not reflect, refract, or polarize like light, and was unaffected by magnetic fields, indicating a new type of radiation.
- The radiation was later named 'uranic rays' and recognized as a fundamental property of matter.
- Marie Curie and Pierre Curie further investigated radioactive materials, isolating radium and advancing the understanding of radioactivity.
- Their work led to the identification of new radioactive elements and the development of radioactivity as a scientific field.
- In 1903, Becquerel and the Curies were awarded the Nobel Prize in Physics for their pioneering contributions.
- The discovery of radioactivity transformed physics, medicine, and our understanding of atomic science.
Chapters
- 00:00Röntgen's Discovery of X-rays
- 01:32Henri Becquerel's Background and Early Work
- 03:05Becquerel's Uranium Salt Experiments
- 04:17Unexpected Discovery in the Dark Drawer
- 05:07Verification and Further Testing
- 06:12Properties of the New Radiation
- 07:16Testing Reflection, Refraction, and Polarization
- 08:22Magnetic Field Experiments and Radiation Classification
- 09:30Marie Curie's Contributions and Radium Isolation
- 18:44Recognition and Nobel Prize Award
Full Transcript — Download SRT & Markdown
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Röntgen was experimenting in a darkened lab when he noticed something odd. A screen coated with fluorescent material began glowing, even though it wasn't in the path of either the electron beam or the fluorescent glass.
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Something unseen seemed to be escaping the tube. Röntgen kept investigating, and what he found was unlike anything anyone had seen before. This mysterious new radiation could pass through paper, wood, and even flesh, but not through denser materials like bone or lead. He
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decided to call the new radiation X-rays, with the X as a nod to a placeholder in a mathematical equation. Röntgen's discovery of X-rays changed physics, medicine, and the way scientists thought about invisible forces entirely.
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However, his discovery also had an unintended side effect. He indirectly opened the door to an entirely new discovery. Just one year later, that world of a new invisible and mysterious kind of radiation would lead a French physicist to discover something even
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more surprising than X-rays. Antoine Henri Becquerel was born in 1852 into a family where science was practically an inheritance. His grandfather, Antoine César Becquerel, was a pioneer in the study of luminescence. His father, Edmond Becquerel, discovered both the
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photovoltaic effect and thermionic emission. The entire environment Henri grew up in pointed him along a path towards physics. He was raised in Paris, studied there, and obtained his doctorate in 1888 from the University of Paris with a thesis on the plane polarization of
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light with a phenomena of phosphorescence and absorption of light by crystals. Eventually, Becquerel moved beyond his family name and became a respected scientist in his own right, especially in the study of light, crystals, and electrical phenomena.
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By the 1890s, he was already established as a prominent physicist at the Muséum national d'histoire naturelle. When Wilhelm Röntgen announced his discovery of X-rays in 1895, scientists across the world were stunned. Here was a new kind of invisible radiation that could pass
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through solid objects and produce photographic images. Becquerel was among those fascinated by the new phenomenon, and like many others, he began wondering whether there might be a connection between X-rays and other forms of light.
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He was especially interested in phosphorescence, the kind of glow some materials give off after being exposed to sunlight. He knew from prior experience that certain uranium compounds, especially phosphorescent ones, might possibly produce X-rays of their own since the phosphorescent glass
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at the end of a cathode ray does the same. So, he designed an experiment as follows. He exposed uranium salts to the sun for long periods of time, and wrapped photographic plates in black paper to block out visible and UV light.
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Then, he placed the uranium salts on top of the plates and observed whether the plates were exposed through the paper.
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If the uranium salt produced X-rays, a shadow image of the uranium would show up on the developed plates. When he unwrapped the plates, he did, in fact, find shadows of the uranium salts embedded into them, seemingly confirming Becquerel's theory that phosphorescent
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materials produce X-rays. At first, the setup seemed like a promising experiment. However, it served more so as a red herring, leading Henri on a path towards a link between sunlight, phosphorescence, and X-rays. He reported his results at the French Academy of
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Sciences meeting on February 24th, 1896. Becquerel immediately wanted to continue his experiments with uranium salts and sunlight, but unfortunately, the weather was overcast for the next several consecutive days.
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So, waiting out the weather, Henri placed the uranium salts and photographic plates in a drawer to wait for the next opportunity.
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There wasn't another sunny day until March 1st, about a week later. And when that day came, Henri took his salts and paper-covered plates back out of the drawer.
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But, instead of beginning his experiments right away, Henri decided to develop the plates that were sitting in the drawer during the few days. Why he did this is still unknown. Based on his context, he shouldn't have expected to
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see anything on the developed plates, as there was no light in the dark drawer to cause any phosphorescence or X-ray emissions.
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One theory suggests he was pressured to report results at the next day's meeting. Another suggests he was simply curious.
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Regardless, when he developed the plates that had been sitting in the dark for several days, what he found would be even more shocking and revolutionary than what he was looking for.
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When Becquerel developed the plates, he found a strong image impressed through the black paper.
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Something from the uranium had reached the plates despite not being exposed to any sunlight.
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Becquerel immediately became enthralled in this revelation and set out to explain how such an event could happen.
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First, he repeated the test to make sure it wasn't a one-off and saw the exact same thing happen again.
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He then compared different uranium compounds, starting with his initial uranium salt, potassium uranyl sulfate, and then running tests with compounds like uranyl sodium sulfate, uranyl ammonium sulfate, and uranium nitrate. He also tried several compounds that didn't contain uranium, such as
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zinc sulfide and calcium sulfides. Interestingly enough, in his initial tests, Becquerel found blue calcium sulfide and blue-green calcium sulfide to produce the strongest images.
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As Becquerel himself stated, "The two blue and greenish-blue luminous calcium sulfides gave very energetic actions, the most intense that I have yet obtained in those experiments." As tests continued, though, he noted that the calcium sulfides became inert. The
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probable cause for the previous pictures showing up on the plates was theorized to be uranium contamination.
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Eventually, Becquerel noticed from his comparisons that the strength of the effect depended neither on whether the material was glowing nor how it was chemically combined. What mattered was the amount of uranium present. He repeated tests with thicker paper over
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the plates and even with 0.02 mm thick aluminum sheets. Still, the effect seemed to ionize air and discharge electroscopes.
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At first, Henri did not fully realize how important his discovery was. He still thought in terms of familiar radiation like phosphorescence and X-ray-like rays.
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He thought he'd found another radiation like light and therefore attempted to run it through reflection, refraction, and polarization tests similar to how Röntgen did with X-rays.
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He aimed the rays at mirrors and polished metal surfaces to see if they would bounce back, but they did not show any form of reflection. Next, he placed prisms and glass lenses in the path of the radiation, hoping to bend the rays
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into a spectrum or focus them, but he ultimately found refraction to be minimal or undetectable.
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Finally, he passed the rays through nickel prisms and polarizing crystals and at first did in fact seemingly find evidence of polarization.
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A few months later, however, this was shown to be an experimental mistake. Lastly, he tested if and how the radiation is affected by magnetic fields. He did this by placing some form of uranium in a collimator to force the
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radiation in one narrow direction. Then, once the rays exited the collimator, he had them travel through a magnetic field in which, on the other side, resided three photographic plates.
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If the magnetic field affected the rays, they would appear on either this plate or this plate, depending on how the rays were charged, positively or negatively.
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If they showed up on this plate, they were not charged at all and were unaffected by the magnetic.
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So, not only was this radiation not behaving like light, but it also seemed to react to magnetic fields in an unpredictable way. To make sense of his data, he split up the radiation into three classes based on their charge. He
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called the three classes positive, negative, and electrically neutral. Upon further months of experimentation, he noticed the radiation never diminished in intensity and eventually dubbed the rays uranic rays, basing them off the element they emitted from.
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Henri initially reported his results on March 1st, the very day he discovered the phenomenon. He published a short memoir on the phenomenon the very next day and, [music] by May of that year, had several papers denoting key characteristics of uranic rays.
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Despite all these new findings, Becquerel did [music] not pursue the topic much further after this.
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He believed the phenomenon was a specific [music] property of uranium and therefore had limited applications and potential. This is where the door was left open for a husband and wife duo to change the world's view on uranic rays
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and bring radioactivity [music] to the mainstream. Marie Skłodowska arrived in Paris in 1891 with nothing but ambition.
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She was a Polish student in a foreign country, scraping by on tutoring, and determined to earn advanced degrees in physics and mathematics at the Sorbonne.
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By 1897, she had married Pierre Curie, who at this point in time was a professor of physics at the Municipal School of Physics and Industrial Chemistry in Paris, and the two had a young daughter named Irene. Marie's next
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step was finding a topic for a doctoral thesis in physics. She wasn't looking for something fashionable. Rather, she was looking for a problem that was important, but not already claimed by the big names of French science. Two recent discoveries
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mainly stood out: Röntgen's X-rays and Henri Becquerel's uranic rays. X-rays were already attracting attention across Europe, but Becquerel's rays, they were just sitting there, proven to exist, but barely explored. And so, she decided her thesis would be about the
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strange continuous spontaneous radiation pouring out of uranium salts. Becquerel's experiments in 1896 had consisted mainly of the use of photographic plates.
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They worked, but they were slow, messy, and hard to quantify. Marie decided to take a different approach. She asked, "If these rays can discharge an electroscope, could she turn that into a precise measurement?" As fate would have it, Pierre and his brother, Jacques, had
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built an extremely sensitive device, a quartz electrometer, to measure very tiny electric charges. Marie realized this instrument was exactly what she needed to answer her question. If she could watch how fast a known charge leaked away in the presence of a sample,
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she could assign a number to how active the substance was. That was inevitably going to be the key shift in the study of radioactivity. She started systematically with uranium.
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Metal uranium, powdered uranium, and different uranium salts, most of them in different physical states.
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Her routine looked quite simple. She would place a sample in a small chamber between two metal electrodes, apply a known voltage, and then use the quartz electrometer to watch how fast the charge leaked away as the air became
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ionized. Faster discharge meant stronger radiation. After a series of tests, she confirmed and strengthened Becquerel's result. The intensity of the effect depended only on the amount of uranium present and not [music] any other factor. Then, she widened the survey.
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She began testing other elements and minerals and stumbled across a very similar phenomenon with the element thorium. Around the same time in Germany, a crystallographer named Gerhard Carl Schmidt had also discovered that thorium emits radiation. He did so,
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however, in a similar manner to how Becquerel did with uranium, using photographic plates, testing refraction, reflection, and polarization, and testing it with magnetic fields.
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Schmidt, though, did not make a distinction that perhaps this is a fundamental property of matter, but constrained it to thorium itself, just how Becquerel did to uranium.
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On top of that, Marie Curie published a three-page note regarding thorium [music] on the 12th of April in 1898.
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Schmidt sent out an 11-page article to be published on the 24th of March a few weeks prior, but it wasn't published until the 23rd of April, 11 days after Marie's publication.
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The publication date, along with the following discoveries made by Marie and Pierre Curie, is why they get credit for this step in radioactivity's history over Schmidt, but both made the thorium discovery independently at around the same time.
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From her discovery with thorium, Marie drew a bold conclusion. This property of uranium and thorium was actually a property of certain elements themselves, rather than a unique characteristic of one specific element. The final disc- discovery she made in her first stints
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with uranium was arguably the most crucial and warranted much further investigation. Some natural minerals, [music] such as pitchblende, a black mineral known to contain uranium, were recorded to be far more active than their uranium and thorium content alone
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could explain. From her observations with pitchblende, she concluded that it must contain something else, some unknown substance even more active than uranium itself. To investigate pitchblende further, she brought in her husband, Pierre, and together they turned their tiny
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makeshift [music] laboratory into an improvised chemical factory that led to the discovery of two new radioactive elements and the coining of a new term that would shape the science of the coming century.
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Pierre himself was fascinated by Marie's pitchblende idea. In early 1898, he joined her project, bringing his experience in precision physics to add They also obtained the assistance [music] of Gustave Bémont, the senior chemist at the university in which
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Pierre taught, and also an expert in chemical separation schemes. Together, they decided on a simple strategy.
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Chemically break pitchblende apart, fraction by fraction, and use the electrometer as an activity detector to track where the strongest radiation came from. They obtained batches of pitchblende and began processing it in their makeshift lab, a drafty old shed
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behind Pierre's school. Step by step, they treated the mineral with acids, precipitated different chemical groups, and separated out fractions containing elements like lead, copper, arsenic, bismuth, and barium. After each separation, they measured the activity of the fraction. If activity went up,
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they knew they were getting closer to finding the unknown substance. If it went down, [music] they could rule it out. One fraction they extracted that behaved chemically almost identically [music] to bismuth stood out entirely.
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Its activity was far higher than could be explained by any trace of uranium or thorium it might contain. By concentrating that bismuth-like fraction over and over, Marie and Pierre obtained a substance that they reported to be roughly 300 times more active than
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uranium. On the 18th of July in 1898, they published a short paper announcing [music] that this highly active material behaved chemically like a new metal very similar to bismuth. They didn't have enough of the material to get it
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atomically weighed, so they didn't have the concrete evidence [music] to prove they had found a new element, but they had many other reasons to conclude that they had. If its existence were to be confirmed in the future, they proposed
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to name it polonium, after Marie's homeland of Poland. [music] It is also in this July 1898 work that the Curies first used the term radioactivity in print, giving a name to the phenomenon Marie had been measuring.
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The bismuth-like fraction of pitchblende was not the end of the story. The Curies also noticed that a different fraction, one that behaved chemically similar to barium, was strongly radioactive as well.
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Following the same logic, they repeatedly purified and recrystallized the barium fraction, each time measuring and sorting the products by radioactivity. The more times they recrystallized and separated the products, the stronger the radiation became.
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And so, on the 26th of December in 1898, the Curies and Bémont informed the French Academy of Sciences that they had strong evidence for a second new element, chemically similar to barium, but vastly more radioactive.
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They named the second radioactive element radium, from the Latin radius, meaning ray. To actually prove beyond doubt that these two hyper-radioactive materials were new elements though, they would have to scale up massively.
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In the following years, the Curies arranged to obtain a massive amount of pitchblende from the Joachimsthal mine in Bohemia.
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Their waste material was far cheaper than fresh pitchblende, but still full of these two seemingly new elements. In their rough shed, they boiled enormous amounts of residue with acids in a giant iron cauldron. They stirred, filtered, crystallized, and recrystallized over
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and over. It was grueling, dirty, and repetitive work, with Marie usually stirring the heavy boiling mixtures herself while Pierre taught at university.
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After processing more than a ton of material, Marie eventually obtained about 0.1 g of pure radium chloride, a tiny amount, but enough to determine an atomic weight and to show its brilliant blue-white glow.
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They weren't able to obtain polonium in this manner though, and atomically weighing polonium didn't come until around 1910 after concepts such as half-life and radioactive decay were understood much better, allowing them to focus on polonium more clearly.
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Regardless, the Curies' 1898 papers were enough evidence to convince most of the scientific community of the existence of the two radioactive elements. Measuring the atomic weights was merely a reassuring nail [music] in the coffin.
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The early history of radioactivity was a relay of clues passed [music] from one scientist to the next. Röntgen first showed that invisible penetration radiation even existed at all with his discovery of x-rays.
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Becquerel then stumbled onto uranium's spontaneous radiation and slowly realized it didn't need light, phosphorescence, or any external trigger. Schmidt quietly added thorium to the list, but still treated it as a quirk of that element. Marie Curie, however, flipped the script entirely.
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[music] She measured the effect precisely, showed it was a new property of certain atoms themselves, and with Pierre and Bémont dubbed the phenomenon radioactivity, and used the idea [music] to uncover polonium and radium from tons of rock.
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The significance in Becquerel and the Curies' accomplishments was recognized in 1903 when the Nobel Prize in Physics was awarded jointly to Henri Becquerel in recognition of the extraordinary services he has rendered by his discovery [music] of spontaneous radioactivity,
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and to Marie and Pierre Curie in recognition of the extraordinary services they have rendered by their joint research on the radiation phenomena discovered by Professor Henri Becquerel.
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By the turn of the 20th century, the story had shifted from weird rays from a few [music] elements to a radically new picture of matter where atoms were no longer inert specks, but rather for some deep reservoirs [music] of hidden energy
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and unstable nuclei waiting to be discovered and used in countless new applications in the [music] 20th and 21st centuries.
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If you enjoyed this video, please consider liking and subscribing. Click here if you want to see more scientific progress [music] made during this time period. Thank you for watching and I will see you in the next video.
Topics:radioactivityHenri BecquerelWilhelm RöntgenX-raysuraniumMarie CuriePierre Curieradiumphysicsscientific discovery
