Some materials are phosphorescent, meaning that they can absorb light energy from the Sun, and then re-emit this energy later. Many objects people use today are phosphorescent, including glow-in-the-dark plastics. Some plastic objects have phosphorescent materials in them, which causes them to glow after they are exposed to light. In 1896, one year after x-rays were discovered, scientist Henri Becquerel hypothesized that uranium might work in this way as well. So he exposed a sample of uranium to sunlight, and then was pleased to discover that the uranium continued to emit energy even after it was taken out of the light. However, after several straight days of cloudy skies over his laboratory, Becquerel was shocked to discover that the energy kept coming from the uranium, even without sunlight. What had Becquerel accidentally discovered?

Explanation:

After the Becquerel came to know the discovery of X-ray by Roentgen  Becquerel began looking for a connection between the phosphorescence  and the newly discovered X-rays. His thought was that phosphorescent uranium salts might absorb sunlight and reemit it as x rays.

To experiment this  he exposed a sample of uranium to sunlight, and then was pleased to discover that the uranium continued to emit energy even after it was taken out of the light. However, after several straight days of cloudy skies over his laboratory, Becquerel was shocked to discover that the energy kept coming from the uranium,  

He reported that the uranium salts emitted radiation without any stimulation from sunlight.  He did further tests to confirm that sunlight was indeed unnecessary, that the uranium salts emitted the radiation on their own.

Initially he thought the effect was due to particularly long-lasting phosphorescence, but he soon discovered that non-phosphorescent uranium compounds exhibited the same effect. After that he announced that the element uranium was indeed what was emitting the radiation.  

At first he also believed his rays were similar to x-rays, but his further experiments showed that unlike x-rays, which are neutral, his rays could be deflected by electric or magnetic fields which was against his expectations. Finally “Radioactivity” term was given to describe this new phenomenon.

This story of becquerel is well known example of accidental discovery.

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