In Becquerel’s initial experiment, in which he exposed uranium salt crystals to sunlight, the photographic paper became exposed even though it was shaded from the sun by a dark cloth. The explanation that he gave was that the crystals were emitting light without any external light source to stimulate or excite them. He called this phenomenon as "radioactivity'". Further, more and more experiments were done to analyse actually what happened between the crystals and the light
