In 1896, French physicist Henri Becquerel was testing whether sunlight-excited uranium salts would emit penetrating rays, similar to the recently discovered X-rays. Overcast weather forced him to store his prepared uranium salts and photographic plates in a dark drawer for several days before testing them. What did he find when he developed the plates, and what did this reveal?
- The plates showed a sharp, exposed image of the uranium salts even though they had never been exposed to sunlight, revealing that the uranium itself emitted radiation spontaneously, without needing an external energy source
- The plates showed no image at all, confirming his original hypothesis that sunlight was required to trigger the uranium salts' emission of penetrating rays
- The plates showed a faint, blurred image that Becquerel attributed to residual light leaking into the drawer through a gap in its lining
- The plates showed a sharp image only on the samples that had briefly caught morning sunlight before being stored, supporting his sunlight-triggered hypothesis
Why A? And why not the others?
Correct answer: A. The plates showed a sharp, exposed image of the uranium salts even though they had never been exposed to sunlight, revealing that the uranium itself emitted radiation spontaneously, without needing an external energy source
Becquerel expected the darkness to mean no reaction at all, since his hypothesis was that sunlight-induced fluorescence was needed to trigger the emission; instead the plates showed a sharp, fully exposed image of the uranium salts despite days of total darkness, which meant the uranium was emitting penetrating radiation continuously and spontaneously, independent of any outside light source. That accidental result overturned his own hypothesis and is now recognized as the discovery of radioactivity, for which he shared the 1903 Nobel Prize in Physics with Marie and Pierre Curie. The option describing no image at all is wrong because it describes exactly the result his sunlight hypothesis predicted, which is the opposite of what actually happened. The option describing light leaking into the drawer is wrong because it would only explain a faint, diffuse fog, not the sharp, clearly defined image he observed. The option describing sunlight-exposed samples alone showing an image is wrong because it was the plates kept fully in the dark, with no sunlight exposure whatsoever, that produced the revealing result.
Source: Britannica: Henri Becquerel