Chance discovery on a cloudy day

A cloudy day in the beginning of March 1896 paved the foundation of a wide range of contemporary uses of spontaneous radiation, when French physicist Henry Becquerel quite accidentally discovered radioactivity — a moment of serendipity in the world of science.

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Though Becquerel was working to verify his postulation that uranium salts could absorb solar energy and re-emit rays, he observed spontaneous radiations from a crystal of uranium, placed on photographic plates in a drawer out of frustration due to cloudy weather.

As many as 129 years after the discovery, radioactivity is used extensively, primarily in medical imaging, cancer treatment, industry, research in geology and archaeology, besides radiometric dating for determining the age of ancient objects and artefacts. It is also used as a source of heat for satellites. However, uncontrolled exposure to radioactivity can cause damage in human tissues, animals, plants and materials.

Becquerel was born on December 15, 1852, in Paris and died on August 25, 1908. He joined a line of distinguished physicists and held the chair of applied physics at the National Museum of Natural History in Paris. It was during his studies of fluorescence and phosphorescence that he emerged as the discoverer of the phenomenon of radioactivity. His keen interest in uranium compounds and skill in photography facilitated the observance of the effect of spontaneous emission of radiations on photographic plates.

Prior to his discovery, the scientific community was intrigued with the discovery of new types of invisible radiation capable of penetrating through black paper. Wilhelm Conrad Roentgen had then found that the specially designed Crookes Tubes he had been using to study cathode rays, emitted a new kind of rays, later known as X-rays.

Becquerel came to know about Roentgen’s discovery during a meeting of the French Academy of Sciences in January 1896, only four fortnights ahead of the discovery of radioactivity. He started exploring the chances of a probable link between X-rays and phosphorescence.

To verify his idea, Becquerel wrapped photographic plates in black paper to prevent sunlight from reaching these plates and placed crystals of uranium salts on the wrapped plates. Outline of the crystals was seen when the plates were developed, an observation that prompted him to infer that phosphorescent uranium salts absorbed sunlight and emitted rays similar to X-rays. He planned to continue his experiments, but the weather turned cloudy. Thinking that he couldn’t continue his research without sunlight, Becquerel put the uranium crystals and photographic plates in a drawer. Contrary to his expectations to see a weak image on the plates after developing, a clear image of the crystal was found.

The baton of study on radioactivity was passed to Marie Curie and Pierre Curie, who figured out how to measure the intensity of radioactivity. Though radioactivity was discovered by Becquerel, Marie Curie coined the term ‘radioactivity’ to describe the phenomenon. The term was explained as ‘spontaneous transmutation of elements’ by Rutherford and Frederick Soddy in 1902.

Becquerel and the Curies shared the 1903 Nobel Prize for their work on radioactivity.