Henri Becquerel discovered radioactivity in 1896 almost by accident. While experimenting with uranium salts and their effect on photographic plates, he discovered that the plates were exposed even when not in direct sunlight, showing that uranium emitted radiation without an external energy source. This led to his discovery of radioactivity, the spontaneous emission of radiation by a material without external energy. For this discovery, which helped establish the field of nuclear physics, Becquerel shared the 1903 Nobel Prize in Physics with the Curies.

1. Henri Becquerel

2. His Life Antoine Henri Becquerel was born on December 15, 1852 in Paris, France. He entered the Polytechnic in 1872, then the government department of Pontset-Chaussees in 1874, becoming ingénieur in 1877 and being promoted to ingénieur-en-chef in 1894. In 1892 he was appointed Professor of Applied Physics in the Department of Natural History at the Paris Museum. He shared the 1903 Nobel Prize for physics for the discovery of spontaneous radioactivity with the Curies .

3. Early Work His early work was with the plane polarization of light, phosphorescence, and the absorption of light by crystals. He also worked with terrestrial magnetism.

4. The Experiment He chose to work with potassium uranyl sulfate, K 2 UO 2 (SO 4 ) 2 , which he exposed to sunlight and placed on photographic plates wrapped in black paper . When he developed the plates, they contained the image of the uranium crystals. This led him to the conclusion that the sun's energy was being absorbed by the uranium which then emitted X rays.

5. The Experiment (continued) But due to the weather, Becquerel was unable to expose more photographic plates, so they were placed into a drawer along with the uranium. By chance, on March 1 st 1896, he decided to develop the plates that had been put in the drawer. He had expected only a faint image of the uranium to appear, but to his surprise, the image was clear. That meant that the uranium emitted radiation without external energy. He had discovered radioactivity.

6. Radioactivity The spontaneous emission of radiation by a material, without external energy. The SI unit for radioactivity, the Becquerel (Bq) was named after him.

7. Other Contributions In 1899 and 1900, he measured the deflection of beta particles, which are essential parts of the radiation in both electric and magnetic fields. From the charge to mass ratio, he showed that the beta particle is the same thing as Joseph John Thomson's recently identified electron.

8. Other Contributions (continued) Another discovery that the active substance in uranium, uranium X, lost its radiating ability in time, while the uranium, though inactive when freshly prepared, eventually regained its lost radioactivity. When Rutherford found the same decay with the element thorium, it led him to create the transformation theory of radioactivity.

9. Other Contributions (continued) His last major contribution was the physical effect of radiation. His report of the burn caused by radium when placed in his vest pocket, inspired research by physicians, which eventually led to medical use.

10. Bibliography http://www.nobel.se/physics/laureates/1903/becquerel-bio.html http://www.accessexcellence.org/AE/AEC/CC/historical_background.html http://en2.wikipedia.org/wiki/Henri_Becquerel http://www.nobel-winners.com/Physics/antoine_henri_becquerel.html