One of the most significant scientific discoveries of the 19th century was made by Antoine Henri Becquerel on this day in 1896. In the course of conducting experiments on uranium salts, Becquerel inadvertently created the conditions that lead to the discovery of natural radioactivity, making him one of the founding fathers of nuclear physics.
Born in Paris in 1852, Becquerel came from a distinguished line of scientists. His grandfather and father were both respected physicists, and both occupied the prestigious physics chair at the National Museum of Natural History. Becquerel studied engineering at the École Nationale des Ponts et Chausses (The National School of Bridges and Roads), becoming an engineer in 1877. But despite his qualification, he continued to pursue an interest in physics, and in 1888 attained a doctorate for his study of light absorption in crystals.
On completing his doctoral thesis, Becquerel continued his research into light and its properties, and it was through this line of scientific enquiry that his important observation was made. He was interested in the discoveries that had been made by fellow physicist Wilhelm Röntgen, who in 1895 had identified x-rays. Röntgen had noted in the summary of his research that a type of phosphorescence occurred in the vacuum tube where the x-rays were produced. Becquerel was keen to establish where this phosphorescence emanated from.
He hypothesised that the phosphorescence occurred as a result of exposure to light, and devised an experiment that would show this, using uranium-rich salt crystals. The uranium was placed on top of a photographic plate that had been wrapped in paper to prevent exposure to light. Becquerel suggested that when the uranium crystals came into contact with light, they would emit a ray that would show up on the photographic plate. But on the day of the experiment, the light conditions were unfavourable, so Becquerel placed all the apparatus in a dark drawer, and waited for another day.
When he returned to the experiment, Becquerel discovered, to his surprise, that despite the lack of light, distinct images had appeared on the photographic plate that could only have been caused by the uranium salt crystals. Even without light, the uranium had spontaneously emitted some sort of energy that had the ability to physically affect another body.
Soon after his chance discovery, Becquerel announced to the scientific world that he had identified a ‘penetrating’ ray, which would soon become known as radioactivity. Expanding on the theories developed in Becquerel’s experiments, two of his students, Marie and Pierre Curie, made further investigations and discovered that similar rays were emitted by other elements. For their pioneering research into radiation, Becquerel and the two Curies shared the Nobel Prize for Physics in 1903.
Perhaps as a result of his constant exposure to radioactive materials, Becquerel died only five years later, at the age of 55. In recognition of his contribution to the field of nuclear physics, the basic unit of radioactivity has become known as the becquerel.
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Photo Caption: French physicist Henri Becquerel in 1900.