Becquerel discovered radioactivity in 1896 purely by accident While
studying the fluorescence and phosphorescence of compounds irradiated
with visible light, Becquerel observed an interesting phenomenon After
illuminating some pieces of uranium-potassium sulphate with visible
light, he wrapped them in black paper and separated the package from a
photographic plate by a piece of silver When, after several hours of
exposure, the photographic plate was developed, it showed blackening
due to something that must have been emitted by the compound and
was able to penetrate both black paper and the silver