Henri Becquerel, a French physicist, discovers radioactivity in 1896.

In Becquerel's early career, he became the third in his family to occupy the physics chair at the Muséum National d'Histoire Naturelle in 1892.

Later on in 1894, Becquerel became chief engineer in the Department of Bridges and Highways before he started with his early experiments.

Becquerel's earliest works centered on the subject of his doctoral thesis: the plane polarization of light, with the phenomenon of phosphorescence and absorption of light by crystals.

Early in his career, Becquerel also studied the Earth's magnetic fields.

Becquerel's discovery of spontaneous radioactivity is a famous example of serendipity, of how chance favors the prepared mind.

Becquerel had long been interested in phosphorescence, the emission of light of one color following a body's exposure to light of another color.

In early 1896, there had been a wave of excitement following Wilhelm Conrad Röntgen's discovery of X-rays on January 5.

Learning of Röntgen's discovery from earlier that year during a meeting of the French Academy of Sciences had caused Becquerel to be interested, and he soon "began looking for a connection between the phosphorescence he had already been investigating and the newly discovered x-rays" of Röntgen, and thought that phosphorescent materials, such as some uranium salts, might emit penetrating X-ray-like radiation when illuminated by bright sunlight.

By May 1896, after other experiments involving non-phosphorescent uranium salts, he arrived at the correct explanation, namely that the penetrating radiation comes from the uranium itself, without any need for excitation by an external energy source.

There follows a period of intense research into radioactivity, including the determination that the element thorium is also radioactive and the discovery of additional radioactive elements polonium and radium by Marie Skłodowska-Curie and her husband Pierre Curie.

The intensive research of radioactivity leads to Becquerel publishing seven papers on the subject in 1896.

His other experiments allow him to research more into radioactivity and figure out different aspects of the magnetic field when radiation is introduced into the magnetic field.