The phenomenon of radioactivity was discovered in 1896 by Henry Becquerel while he was studying the properties of a uranium salt.

Henry Becquerel had kept a photographic plate in his desk.The plate was wrapped with thick black paper to prevent its exposure to light.By chance he kept the Uranium salt on the plate for two days.Then when he developed photographic plate , he found that the plate was affected.As there was no other light to affect the photographic plate he concluded that the Uranium salt must have emitted invisible radiation which was capable of penetrating through thick black paper to reach the photographic plate.

This property was later given the name Radioactivity by Madam Curie who found that Thorium also possesses the same property

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