In late 1896, Henri Becquerel placed a sample of uranium salts on wrapped photographic plates. To his surprise, he discovered that the plates had been exposed—blackened by an invisible emissions coming from the uranium.

Building on his discovery, Marie and Pierre Curie investigated these "Becquerel rays" and found that several other elements emitted similar radiation. It was the Curies who coined the term radioactivity to describe this phenomenon. Their relentless work to isolate the source of this radiation led to the discovery of two new elements: radium and polonium. In 1903, Henri Becquerel, Marie Curie, and Pierre Curie were jointly awarded the Nobel Prize in Physics for their groundbreaking achievements.

This marked the beginning of our modern understanding of radioactivity—the spontaneous emission of particles, radiation, or energy from unstable atomic nuclei. The video below explores what radioactivity is and examines the physics that underpins it.