Planck Curve:

The quantum theory of absorption and emission of radiation announced in 1900 by Planck ushered in the era of modern physics. He proposed that all material systems can absorb or give off electromagnetic radiation only in "chunks" of energy, quanta E, and that these are proportional to the frequency of that radiation E = h. (The constant of proportionality h is, as noted above, called Planck's constant.)


The Austrian physicist Josef Stefan found in 1879 that the total radiation energy per unit time emitted by a heated surface per unit area increases as the fourth power of its absolute temperature T (Kelvin scale). This means that the Sun's surface, which is at T = 6,000 K, radiates per unit area (6,000/300)4 = 204 = 160,000 times more electromagnetic energy than does the same area of the Earth's surface, which is taken to be T = 300 K. In 1889 another Austrian physicist, Ludwig Boltzmann, used the second law of thermodynamics to derive this temperature dependence for an ideal substance that emits and absorbs all frequencies. Such an object that absorbs light of all colours looks black, and so was called a blackbody.



It was the shape of Wien's radiative energy distribution as a function of frequency that Planck tried to understand. The decrease of the radiation output at low frequency had already been explained by Lord Rayleigh (John William Strutt) in terms of the decrease, with lowering frequency, in the number of modes of electromagnetic radiation per frequency interval. Rayleigh assumed that all possible frequency modes could radiate with equal probability, following the principle of equipartition of energy. Since the number of frequency modes per frequency interval continues to increase without limit with the square of the frequency, Rayleigh's formula predicted an ever-increasing amount of radiation of higher frequencies instead of the observed maximum and subsequent fall in radiative power. A possible way out of this dilemma was to deny the high-frequency modes an equal chance to radiate. To achieve this, Planck postulated that the radiators or oscillators can only emit electromagnetic radiation in finite amounts of energy of size. At a given temperature T, there is then not enough thermal energy available to create and emit many large radiation quanta. More large energy quanta can be emitted, however, when the temperature is raised.

Excerpt from the Encyclopedia Britannica without permission.