Here's the breakdown:

* Becquerel was studying fluorescence, a phenomenon where certain materials emit light after being exposed to ultraviolet light.

* He believed that fluorescence might be related to X-rays, which were a recent discovery at the time.

* He wrapped a photographic plate in black paper to protect it from light and placed a uranium salt on top.

* He then exposed the setup to sunlight, expecting the uranium to fluoresce and expose the plate.

* To his surprise, the plate was exposed even when it wasn't exposed to sunlight.

* This led him to conclude that the uranium was emitting some kind of invisible radiation that could pass through the black paper and expose the photographic plate. This radiation was later named radioactivity.

So, the photographic plate was a crucial tool in Becquerel's discovery.