The crystals emitted something that affected the film the same way light did.
<h3><u>Explanation;</u></h3>
Becquerel discovered the spontaneous emission of radiation from an element which he called radioactivity.
He placed uranium salts on top of an unexposed photographic plate which was wrapped in a desk drawer and when the plate was developed, there was an intense silhouette on the film.
He discovered that a piece of mineral which contained uranium could produce it's image on a photographic plate in the absence of light.