How did Henri Becquerel discover radioactivity in 1896?

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radioactivity Figure 1: Radioactive decay of beryllium-7 to lithium-7 by electron capture (EC; see text).

In 1896, Henri Becquerel discovered radioactivity while investigating whether luminescent materials emitted X rays similar to those recently discovered by Wilhelm Röntgen.

Becquerel hypothesized that phosphorescent crystals might yield X rays when stimulated by sunlight. He placed uranium salts upon a photographic plate wrapped in opaque paper and exposed them to sunlight. Upon developing the plate, he observed silhouettes of the mineral samples, confirming that a penetrating radiation had passed through the paper.

Initially, Becquerel believed this radiation was a form of invisible phosphorescence triggered by sunlight. However, he soon discovered that uranium salts continued to emit this radiation even when kept in the dark and not stimulated by light. He eventually traced this activity to the uranium metal itself, interpreting it as a unique property of the substance. While he initially viewed these "Becquerel rays" as less significant than X rays, his work laid the foundation for the field of nuclear physics, for which he later shared the 1903 Nobel Prize for Physics.1