10.1 Radioactivity

In 1896, French physicist Antoine Henri Becquerel (1852—1908) was experimenting with uranium salts. He hypothesized that the salts, which glow after being exposed to light, produced X-rays while they glowed. To test his hypothesis, Becquerel performed an experiment. First, he wrapped a photographic plate in paper. Then, he placed some uranium salts on the plate and set it outside in the sunlight, which caused the salts to glow. When Becquerel developed the plate, he got a foggy image. At the time, Becquerel thought that X-rays from the salts had penetrated the paper and fogged the plate.

Like any good scientist, Becquerel wanted to repeat his experiment, but a spell of bad weather forced him to wait. In the meantime, he left a wrapped photographic plate and uranium salts in a desk drawer. After several days, Becquerel decided to develop the plate without exposing the uranium to sunlight. To his surprise, he got the foggy image shown in Figure 1A. Later, Becquerel determined that the uranium salts had emitted rays that had never been observed before.

Figure 1 Due to rainy weather, Henri Becquerel postponed his intended experiment with uranium salts. A Without any exposure to sunlight, the salts still produced a foggy image on a photographic plate. B For his discovery of radioactivity, Becquerel shared the 1903 Nobel Prize for Physics with Marie and Pierre Curie.

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