The discovery of radioactivity was purely accidental. It was discovered in1896 by a French Physicist A.H. Becquerel, who was interested at that time in the phenomenon of fluorescence. Fluorescence is the ability of certain substances to transform incident ultraviolet radiation into visible light.
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In one of the drawers of his desk, Becquerel kept a collection of various minerals that he was going to use for his studies. But because of other pressing matters, the collection remained untouched for a considerable period of time. It happened that in the drawer there were also several unopened boxes to photograph something. When he developed the plates, he was disappointed to find that they were badly fogged as if they were previously exposed to some light.
A check on the plates in other boxes in the drawer showed that they too were in the same poor condition. Becquerel could not guess the reason immediately, since all boxes were sealed and the plates inside were wrapped in thick black paper. What could have been the cause of this unexpected fogging ? Could it have something to do with one of the minerals in the drawer ? Yes, Becquerel concluded. So , he performed a series of experiments using a light - tight box, inside which one of the minerals, pitch - blend (uranium ore) and a photographic plate were arranged either in contact with each other or separated by thin sheets of aluminium.
The results obtained clearly showed that the active radiations that affected photographic plates came from the mineral itself. The exposure of the uranium ore to sunlight had no influence what soever on the phenomenon exhibited by it.More about Discover of Radioactivity:Continuing his research with different compounds of uranium, he found that the newly discovered radiations were emitted by uranium element irrespective of its state of chemical combination and physical conditions such as temperature, pressure etc. He named these newly discovered radiations emitted by uranium as 'Becquerel rays' . Later , it was changed to "radioactivity" to designate all such rays given out by several other substances with atomic number greater than 83.
Their nuclei are generally unstable. These unstable nuclei can spontaneously undergo an internal change ejecting particles and electromagnetic radiations in the process called "Radioactivity". The radioactivity is essentially a nuclear phenomenon. Some of the radiations can penetrate easily through thick substances.discovery of radioactivity : ConclusionThe self - disintegrating, spontaneous activity in the form of emission of radiations by some unstable nuclei with atomic number greater than 83 is called natural radioactivity.