**Planck discovers the quantum nature of energy**

1900

In 1899 Max Planck became a professor at the University of Berlin, after nine years at the University of Munich and Kiel University, in Germany. In his research there, he turned to a thermodynamic problem raised by one of his old teachers. The problem was that of a "black body," something that absorbs all frequencies (or wavelengths) of light. When heated it should then radiate all frequencies of light equally -- theoretically. But the distribution of energy radiated in "real life" never matched up with the predictions of classical physics. Several distinguished physicists had created complex equations trying to work it out, but none solved the problem.

Planck was as steeped in traditional physics as his colleagues, but he had an open mind. The older way wasn't working. So he changed one basic assumption: energy, instead of being continuous, comes in distinct particles. These were later called "quanta," from the Latin for "how much?" Though it sounded outlandish, applying this idea to the problem of heated bodies revealed a simple relationship that explained previous puzzles. Planck found that the energy radiated from a heated body is exactly proportional to the wavelength of its radiation. So, a black body would *not * radiate all frequencies equally. As temperature goes up, energy increases and it's more likely that quanta with higher energy will be radiated. So, as an object heats up, the light given off is orange, then yellow, and eventually bluish. The wavelength emitted is a function of the energy times a constant (h), now known as Planck's constant. Though Planck's idea was not immediately believed by most physicists, it is now accepted as one of the fundamental constants in the universe. In fact, Planck himself wasn't sure if it was more than a little mathematics that resolved his own particular problem.

In 1905, Albert Einstein used the theory of quanta to accurately describe the photoelectric effect. In 1913, Niels Bohr incorporated Planck's idea into his revision of model of the atom, resolving inconsistencies that classical physics could not.

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