Who better to kick off this list than perhaps the most famous scientist in the history of the world? Albert Einstein won the Nobel Prize in Physics in 1921 for discovering the cause of the "photoelectric effect." This was a perplexing phenomenon in which atoms, when bombarded with light, emitted electrons. In 1905, Einstein argued that light was divided into discrete packets (which we now call photons). He theorized that, when these light packets struck atoms, electrons in those atoms absorbed them, and, with the extra energy, wrested free of the atoms that bound them.
The fact that light is composed of particles that are absorbed and emitted by atoms was just one of Einstein's many revolutionary discoveries. He also came up with the theories of special and general relativity, and discovered that matter and energy are equivalent (as embodied in the equation E=mc²). A true polymath — within science, at least — he even wrote a paper explaining why the average "meandering ratio" of a river — the ratio of its length to the distance between its source and mouth as the crow flies — is equal to pi.