Henri Poincaré’s interest in X-rays influenced Henri Becquerel before the discovery of radioactivity.
After Wilhelm Röntgen announced X-rays in 1895, Poincaré discussed their apparent connection with fluorescence in cathode-ray tubes. He suggested that fluorescent or phosphorescent materials might be able to produce X-ray-like radiation. Becquerel, who already studied phosphorescence, was prompted to test whether fluorescent uranium salts behaved similarly.
In early 1896, Becquerel placed uranium compounds near photographic plates. Although he initially expected sunlight-triggered fluorescence to be involved, he found that the uranium salts could fog a covered plate even when kept in darkness. The radiation therefore came spontaneously from the uranium, not from an external light source.
Poincaré did not discover radioactivity, and Becquerel’s experiments established the phenomenon. The connection with X-rays was an important starting point, while later work by Marie and Pierre Curie, Ernest Rutherford, and Soddy revealed radioactivity’s broader nature and its role in nuclear transformation.