Radioactivity
While conducting experiments to understand the relationship between X-rays (discovered by the German physician Wilhelm Röntgen) and fluorescence, Henri Becquerel, in 1896, discovered radioactivity. After having carried out an experiment with fluorescent uranium salt crystals on photographic plates covered in black paper and exposed to sunlight, he discovered that the salt had left clear outlines on one of the plates that had stayed in the cupboard, suggesting that they were emitting radioactivity.