Radioactivity was first discovered in 1896 by the French scientist Henri Becquerel whilst working on phosphorescent materials. These materials glow in the dark after exposure to light, and he thought that the glow produced in cathode ray tubes by x rays might somehow be connected with phosphorescence. So he tried wrapping a photographic plate in black paper and placing various phosphorescent minerals on them. All results were negative until he tried using uranium salts. The result with these compounds was a deep blackening of the plate.
However, it soon became clear that the blackening of the plate had nothing to do with phosphorescence because the plate blackened when the mineral was kept in the dark. Also non-phosphorescent salts of uranium and even metallic uranium blackened the plate. Clearly there was some new form of radiation that could pass through paper that was causing the plate to blacken many books state that Becquerel accidentally discovered radioactivity as though his skill as a scientist had nothing to do with it. In actual fact he was a good scientist who deserves full credit for his work .
At first it seemed that the new radiation was similar to then recently discovered x rays. However further research by Becquerel, Madam Curie, Rutherford and others revealed that there are three different types of radiation, and that many other elements apart from uranium are radioactive.
The three types of radiation are alpha rays (α) beta rays (β) and gamma rays (γ)