Radioactivity

Radioactivity was first discovered and investigated in 1896 by Henri Becquerel, who found that uranium salt emitted radiation , which could penetrate through opaque material , after photographic plates ,cause gases to ionize and make certain substance to fluorese.
In 1898, Pierre and Marie curie , detected some radioactivity in the element thorium. They also noted that level of radiation in a naturally occurring uranium was much higher than could be explained by its uranium content, and they were able to isolate two radioactive element ,polorium and radium.