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Students learn that radioactive substances give off three kinds of radiation: alpha, beta and gamma. The third was found by Paul Villard in Paris in 1900, using a magnet, two photographic plates and a strip of lead. This post walks through his two short notes to the French Academy of Sciences, explains why a magnet cannot bend gamma rays, works through an example on gamma-ray energy, and ends with what students can learn from his method.
What was known about radium's rays in 1900?
Radioactivity was still a new field. Villard's own notes in the Comptes rendus of the French Academy, volume 130, show what was known. Part of radium's radiation, the "rayons déviables", could be bent by a magnet. Pierre Curie had also described rays that a magnet did not seem to bend, but that were easily absorbed. Those easily absorbed rays were alpha particles, and they did not stay unbendable for long: the title of Ernest Rutherford's 1903 paper, The magnetic and electric deviation of the easily absorbed rays from radium, tells the outcome.
What did Villard find in his first note?
At the Academy's session of 9 April 1900, Villard presented a note on the reflection and refraction of cathode rays and of the deviable rays of radium, printed in the same volume from page 1010. His photographs had given confusing results. He guessed that radium was also sending out non-deviable rays "less absorbable" than the ones Curie had described.
He tested the guess with a magnet. The beam split into two groups, fully separated after a few centimetres. One bent the expected way. The other went straight down the whole length of the plate. This straight beam darkened a plate 25 cm away through several sheets of black paper and an aluminium sheet, and could even pass through lead 0.2 mm thick. He concluded that the non-deviable part of radium's emission contains "radiations très pénétrantes", very penetrating radiations.
How did his second experiment settle it?
Pierre and Marie Curie then lent him a much more active radium sample. His note of 30 April 1900, in the same volume from page 1178, describes the set-up:
Rays from the radium passed through a slit in a lead bar.
They struck two photographic plates stacked one on top of the other, each wrapped in thick black paper, at a grazing angle, inside a magnetic field.
The top plate showed two beams: one bent and spread out, the other weaker but perfectly straight, with sharp edges.
The bottom plate, reached only by rays that had passed through the top one, showed just the straight beam, as strong and sharp as before. At the far end the rays had crossed more than 1 cm of glass.
A lead strip 0.3 mm thick laid on the top plate wiped out the bent beam but only weakened the straight one.
Villard referred to the straight rays as X-rays emitted by radium, and said the situation was analogous to what happens with Crookes tubes. About 1 cm of glass, he wrote, practically stopped the deviable rays but weakened the others very little.
What are gamma rays?
Encyclopaedia Britannica's entry on the discovery of radioactivity notes that the radiation identified by Villard in 1900 was designated the gamma ray, is not deflected by magnets, and was later shown to be electromagnetic radiation like light or X-rays, with much shorter wavelengths. NCERT's Class 11 chemistry chapter on the structure of the atom describes gamma rays as "high energy radiations like X-rays", neutral, and gives their penetrating power as about 1,000 times that of alpha particles, against 100 times for beta rays.
Worked example: why no bending, and how energetic is a gamma ray?
The force: a magnetic field pushes on a moving charge with F = qvB sin θ. A gamma ray carries no charge, so q = 0 and F = 0. It goes straight, however strong the magnet.
A real gamma ray: the medical isotope technetium-99m emits a gamma ray of 140.511 keV, according to the NuDat nuclear data of the US National Nuclear Data Center.
Its wavelength: λ = hc/E. Using hc ≈ 1,240 eV nm, λ = 1,240 / 140,511 ≈ 8.8 x 10^-3 nm, or about 8.8 picometres.
Compare green light at 500 nm: E = 1,240 / 500 ≈ 2.48 eV. The gamma photon carries about 140,511 / 2.48 ≈ 57,000 times more energy.
Common mistakes to avoid:
Thinking gamma rays are bent a little. With no charge, the magnetic force is exactly zero.
Mixing keV and eV. 140.511 keV is 140,511 eV. Leaving out the thousand makes the wavelength a thousand times too long.
Calling gamma rays particles with mass. They are photons, like X-rays, only more energetic.
The technetium-99m example comes from our post on Emilio Segre and technetium. For the discovery that started it all, read about Henri Becquerel and radioactivity.
What can a student take from how Villard worked?
Treat confusing results as clues. His muddled photographs were the first sign of a new kind of ray.
Turn a guess into a test. He suspected non-deviable rays and checked with a magnet.
Separate the effects. Stacked plates and a lead strip let him measure penetration and bending at once.
Report short and fast. Two notes, three weeks apart, each only two or three pages long.
If the link between wavelength, energy and penetrating power still feels loose, the Fermi Concept Library covers every concept in physics and chemistry and shows what each one builds on.
Next step: find the wavelength of a 1.0 MeV gamma ray and check that it is about 1.24 picometres.
