Whose discovery first revealed radioactivity?

French physicist Henri Becquerel discovered radioactivity in 1896 while studying phosphorescent materials. He found that uranium compounds could fog photographic plates even when kept in the dark, proving they emitted their own penetrating radiation. This unexpected result opened a new era in physics and chemistry. Marie and Pierre Curie later expanded the field by discovering polonium and radium. Radioactivity became central to nuclear physics, medicine, and energy research.