Subsequent to Roentgen's discovery of X-rays, in 1896 a French scientist Henri Becquerel was experimenting with a uranium compound. While investigating the properties of fluorescent minerals, it was Becquerel who discovered that certain types of atoms disintegrate by themselves. When working on the principles of fluorescence, he utilized photographic film to record fluorescence of various minerals when exposed to sunlight.
One of the minerals Becquerel worked with was a uranium compound. The experiment normally consisted of wrapping some photographic film in light proof paper, placing a piece of fluorescent uranium on top of the film, and leaving them in the sun. One day, after preparing the experiment, it was too cloudy to expose his samples to direct sunlight, so he stored the uranium compound and the film in a drawer. A couple of days later, he decided to develop this film anyway, and discovered an image of the uranium sample on the film. Becquerel questioned what would have caused this. He knew he had wrapped the film tightly in light proof paper, so the image was not due to stray light.
In addition, he noticed that only the film that was in the drawer with the uranium compound had an image on it. Becquerel concluded that the uranium compound gave off something invisible that could penetrate heavy paper and affect photographic film. Becquerel continued to test many samples of compounds and determined that the source of the invisible something was the element uranium. This invisible something was named radiation, and it was determined that an element that gives off radiation is a radioactive element. Today, we know uranium as one of the radioactive elements. For his discovery of radioactivity, Becquerel was awarded the 1903 Nobel Prize for physics.