Henri Becquerel discovered that uranium salts emit invisible radiation that can penetrate opaque materials and affect photographic plates. This was a groundbreaking discovery that led to the field of radioactivity.

Here's a breakdown of his discovery:

* Background: Becquerel was studying the phenomenon of phosphorescence, where certain materials glow after being exposed to light. He was interested in whether uranium salts could also emit X-rays, which had recently been discovered.

* The Experiment: He wrapped photographic plates in black paper and placed uranium salts on top. He expected that if the salts emitted X-rays, they would penetrate the paper and expose the plates.

* The Unexpected Result: To his surprise, the plates were exposed even when not exposed to light. This showed that something other than phosphorescence was causing the exposure.

* The Discovery: Becquerel concluded that the uranium salts were emitting a new type of radiation, which he called "Becquerel rays". This radiation had the remarkable ability to penetrate opaque materials and affect photographic plates, even in the absence of light.

This discovery paved the way for further research into radioactivity, leading to the work of Marie and Pierre Curie who identified the radioactive elements polonium and radium.