Physics In The 20th Century

Planck never pursued the implications of this notion, but 5 years later Albert Einstein did, suggesting that light itself was made up not of waves, but of packets of energy (later named photons); the higher the frequency of the light, the more energetic the photon. He then demonstrated how under some conditions, electrons could absorb and emit the energy of photons, and--in a breakthrough that would earn him the Nobel Prize--he used this demonstration to explain what was called the photoelectric effect (the discharge of electrons from matter by the impact of radiation, especially visible light).

Meanwhile, not everyone agreed with Einstein's theory of light-as-particle; the debate would continue for a couple of decades. But even before physicists accepted that light was somehow both wave and particle, Einstein discovered yet another phenomenon. According to Niels Bohr's model of the atom, set forth in a series of papers in 1913, electrons occupy specific orbits around the nucleus, determined by the electrons' energy levels. An electron can absorb only the exact amount of energy needed to kick it from one orbit up to a specific higher one, and it emits a specific amount of energy on dropping from an orbit to a lower one. That explained why atoms of a given gas, such as neon, emit a distinctive pattern of wavelengths, and why vapor discharge lamps such as those based on mercury or sodium have a characteristic color.

Atoms that are in an excited state--that is, when their electrons are in higher-energy orbits--will eventually, and spontaneously, fall back to their lowest, or ground state, giving off stored energy in the process. In a given system of atoms, this spontaneous emission occurs at random, with the emitted photons of energy heading off in random directions. Einstein recognized that if atoms in an excited state encounter photons of light with the right amount of energy (namely, an amount equal to the difference between the lower-energy and higher-energy states), the encounter can trigger a kind of chain reaction of emission that boosts the intensity of the light passing through--as though the electrons, greedy to capture the incoming photons, dropped the ones they already had stored up. Furthermore, the emitted photons all headed in the same direction as the incoming photons. The process is called "stimulated emission."