In 1896, Henri Becquerel was using naturally fluorescent minerals to study the properties of X-rays. He exposed potassium uranyl sulfate to sunlight and then placed it on photographic plates wrapped in black paper. He believed that uranium absorbed the Sun’s energy and then emitted it as X-rays.
This assumption was later proved wrong. Because it was cloudy and overcast in Paris when Becquerel did his experiment, he expected it to fail because uranium needed sunlight as per his belief. However, when he developed the photographic plates anyway, Becquerel found strong and clear images! This proved that uranium emits radiation without an external energy source and thus radioactivity was discovered.
The term radioactivity was coined by Marie Curie, who together with her husband Pierre began investigating the phenomenon discovered by Becquerel. They extracted uranium from an ore and were surprised to find the leftover ore more radioactive than pure uranium. Their conclusion that the ore had more radioactive elements led to the discovery of polonium and radium.
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