Becquerel's early work focused on the polarization of light phosphorescence and terrestrial magnetism. In 1896, he made his greatest discovery: radioactivity. Following a discussion with Henri Poincaré on the recent discovery of X-rays, Becquerel devised an experiment that proved the existence of this naturally occurring force. Although his initial experiments were not successful he came upon evidence of natural radioactivity nearly by accident, when an unexpected reaction occurred in one of his laboratory drawers. Becquerel was awarded the Nobel Prize for Physics in 1903, sharing the honor with Pierre and Marie Curie.