In 1900 Max Planck, professor of theoretical physics in Berlin, found a mathematical formula that described the energy of a radiating substance. He found that the transfer of energy was not continuous, but instead included little packets, or bundles, called quanta. (The rainbow which appears to be a continuous band of color is really made up of little units, or bundles, of light of different energies.)
When any piece of matter is heated, it starts to glow, gets red hot, and white hot at higher temperatures. This excitement of atoms, or molecules, of the matter is called radiation. When black materials are heated, the radiation depends solely upon the temperature. With colored materials, radiation is dependent upon more factors than temperature. Therefore, the radiation emitted by a black material at high temperatures is a suitable object for research because there is only the factor of temperature to consider. The radiation of a black body should be explainable in terms of the laws for radiation and heat. In the late 1800's Lord Raleigh and Sir James Jeans attempted to explain this hypothesis, but they failed.
When Planck attacked this problem, he turned from black-body radiation to radiating atoms. Experimental data on heat and light-radiating atoms led him to devise a mathematical formula to account for the behavior of such radiation energy. Surprisingly, his formula made it seem that atoms contain only discrete quantities, or quanta (quantum is the singular form) of energy, rather than their possessing continuous wave-like energy as classical physics had always assumed. The formula produced a puzzle such as one would face if a car can travel at 50 and at 52 miles per hour, but cannot go at 51 m.p.h.
In 1905, Albert Einstein gave theoretical support for Planck's observations. Einstein's explanation was based on an experiment, the photoelectric effect, which showed that the energy of electrons emitted when light fell on a metallic surface depended, not upon how much light was present, but rather upon the frequency (or wave length) of the light. That is, the size of the radiation units called photons (quantum units) depended upon the rate of vibration of the atoms of the particular material; furthermore, the total energy emitted was related only to this rate, and thus could be expressed mathematically by merely three symbols, one of which was the "constant of proportionality." In honor of Max Planck, this constant was called Planck's Constant. The formula is: E = Hf; where E stands for the total energy of the emitted photon (in erg-seconds); f is the rate at which the radiant energy is vibrating, and H is Planck's Constant. (H is calculated to be 6.62 X 10−27.)
In other words, Planck and Einstein stated that light (and other radiant energy) travels through space, in continuous waves, but is absorbed or emitted in energy packets, or quanta.
Shortly after the quantum theory was first announced, Niels Bohr, a Danish physicist, was able to apply it further to form a new theory about atomic structure. Bohr's original atomic model has been modified, but even the latest theories still agree with the idea of energy quanta.
