Becquerel was awarded the Nobel prize for his discovery of radioactivity. He was researching phosphoresence and decided one day to place the stuff he used in a drawer to keep it out of the sun. He put it on top of a photographic plate (coincidentally) and saw that on the plate the form of the stuff he put on top. Radioactivity was discovered.
Now in Wikipedia one can resd:
As often happens in science, radioactivity came close to being discovered nearly four decades earlier in 1857, when Abel Niépce de Saint-Victor, who was investigating photography under Michel Eugène Chevreul, observed that uranium salts emitted radiation that could darken photographic emulsions. By 1861, Niepce de Saint-Victor realized that uranium salts produce "a radiation that is invisible to our eyes". Niepce de Saint-Victor knew Edmond Becquerel, Henri Becquerel's father. In 1868, Edmond Becquerel published a book, La lumière: ses causes et ses effets (Light: Its causes and its effects). On page 50 of volume 2, Edmond noted that Niepce de Saint-Victor had observed that some objects that had been exposed to sunlight could expose photographic plates even in the dark. Niepce further noted that on the one hand, the effect was diminished if an obstruction were placed between a photographic plate and the object that had been exposed to the sun, but " … d'un autre côté, l'augmentation d'effet quand la surface insolée est couverte de substances facilement altérables à la lumière, comme le nitrate d'urane … " ( ... on the other hand, the increase in the effect when the surface exposed to the sun is covered with substances that are easily altered by light, such as uranium nitrate ... )
So Becquerel's father was already aware of the effect. The effect on photographic plates was enhanced if a uranium salt was placed between the sun-stuff and the plates. How was this interpreted? How could uranium salt enhance this effect on the plates, instead of decreasing it? I guess not in the way his son did. I am sure that Becquerel jr. read his father's book. Was he influenced by this observation his father made? At first sight you would say no, because then he would have done experiments with salts without sunlight from the start. To examine what his father described in his book. Or would he have done these anyhow? Was he unaware of the book maybe? I cannot imagine. Why didn't he try to reproduce the observation made already in 1861 by de Saint Victor? Now of course people back then were still in the dark and it is always east talk in hindsight. Was Bequerel trying to explain it all by electromagnetic radiation only? So the uranium salt put on the phosphoresence stuff was somehow activated by that stuff? Did he think from the start that the guy his father wrote about saw radioactivity? Did he need some incident to make it clear that the uranium salt radiates on his own? This is pure soeculation of course. If he thought uranium salt radiates on its own he might just as well have had put it in the dark on a photographic plate. Did he maybe think a coincidental discovery was more spectacular? On the other hand, doing a conscious experiment to examine if your theory is right demonstrates the fertility of the mind. Was he aware of his fathers writings (testimonies)?