The discovery of radioactivity by Becquerel involved the effect of uranium salts on which material?

• A. Photographic plates
• B. Glass plates
• C. Metal foils
• D. Paper strips

Answer: (A)

Radiation can be detected by how it interacts with photographic emulsions. Becquerel found that uranium salts emitted radiation that could expose a photographic plate wrapped in black paper, causing the plate to develop a foggy image even in the dark. This showed that the radiation itself was recording onto the photographic medium, revealing radioactivity as a spontaneous emission. The other materials—glass plates, metal foils, and paper strips—don’t serve as the same direct, sensitive detector for this kind of exposure, so photographic plates were the crucial tool that let him witness the effect.