Introduction

Although physicists felt comfortable in the late 1800s with their capabilities to describe the macroscopic world, many questions remained unanswered. Lord Kelvin’s lecture on the “Nineteenth-Century Clouds over the Dynamical Theory of Heat and Light,” referred to two specific problems that interested him. Namely, he discussed the lack of physical theory to explain the speed of light, and the failure of physics to explain blackbody radiation. However, there were many other puzzles that were just as baffling.

One challenge was explaining the recently discovered phenomenon of radioactivity. Physicists were perplexed by the idea that energy could be spontaneously generated by matter. The classical view of physics simply couldn’t explain how rocks could spontaneously glow in the dark or fog photographic film without light.

In 1896, French physicist Henri Becquerel learned about the greenish glow produced by recently-discovered X-rays in vacuum tubes. He decided to investigate whether there was any connection between the glow produced by X-rays and naturally occurring fluorescence found in certain minerals. Becquerel had inherited from his father some uranium salts, which fluoresce on exposure to UV light, and which were known to fog photographic plates, even if the plates were stored in dark envelopes. He hypothesized that uranium salts emitted X-rays when “charged” by exposure to sunlight. Much to his surprise, the salts fogged photographic paper, although the day he chose for his experiment was dark and rainy.

Later, Becquerel showed that the rays emitted by uranium were different from X-rays, because they could be deflected by electric or magnetic fields. For his discovery of spontaneous radioactivity, Becquerel was awarded half of the 1903 Nobel Prize for Physics. The other half was awarded to Pierre and Marie Curie for their study of “Becquerel radiation.”

We take a step back in history and discuss atoms and radioactivity. Research in these fields became crucial for taking Planck and Einstein’s quantum from being a mere curiosity in the study of light to its place in our understanding of matter. We will return to proper quantum physics.