Discovering Radioactivity

In 1856, Henri Becquerel was studying the phosphorescence (glowing) of certain minerals when exposed to light. In his experiments, he’d take a mineral sample, place it on top of a heavily wrapped photographic plate, and expose it to strong sunlight. He was preparing to conduct one of these experiments when a cloudy spell hit Paris. Becquerel put a mineral sample on top of the plate and put it in a drawer for safekeeping.

 Days later, he went ahead and developed the photographic plate and, to his surprise, found the brilliant image of the crystal, even though it hadn’t been exposed to light. The mineral sample contained uranium. Becquerel had discovered radioactivity.