Radioactive decay

Henri Becquerel's cloudy week, Paris, March 1, 1896 Inspired by X-rays, Becquerel suspected that glowing uranium salts absorbed sunlight and gave it back out as X-rays. To test it he needed sun, and Paris stayed overcast for several days in late February, so the wrapped photographic plates and uranium sat in a drawer. On March 1 he developed them anyway, expecting a faint image, and found a remarkably clear one. Why he bothered is still debated, but he reported the next day that the salts emitted radiation with no sunlight at all. Marie and Pierre Curie took up the strange rays, and the three shared the 1903 Nobel Prize for work on radioactivity.