The nature of radioactivity was discovered in 1896 byAntoine Henri Becquerel, who studied minerals that incorporate uranium and other radioactive elements. He placed some of these samples in a drawer of his desk along with an uneposed photographic plate and a metal coin. When he developed the photographic plate later, the sihouette of the coin was clearly visible.
From this photograph he concluded that some unknown form of radiation that got around the coin and reached the plate deliverted enough energy to cause the chemical reactions that normally go into photographic development. Becquerel knew that whatever had exposed the plate must have originated in the minerals and travelled at least as fast as that plate. Later on, many scientist worked hard to get the answer and one of the most famous one is Marie Sklodowska Curie.