PLANCK'S RADIATION LAW

a mathematical relationship formulated in 1900 by Max Planck, a German physicist, to explain the spectral-energy distribution of radiation emitted by a blackbody (a hypothetical body that completely absorbs all radiant energy falling upon it, reaches some equilibrium temperature, and then reemits that energy as quickly as it absorbs it). Planck assumed that the sources of radiation are atoms in a state of oscillation and that the vibrational energy of each oscillator may have any of a series of discrete values but never any value between. Planck further assumed that when an oscillator changes from a state of energy E1 to a state of lower energy E2, the discrete amount of energy E1 - E2 is emitted as a photon, or quantum of radiation. The energy of this quantum is equal to the product of the frequency of the radiation, symbolized by the Greek letter nu, n, and a constant, h, now called Planck's constant; i.e., E1 - E2 = hn. Planck's law for the energy El radiated per unit volume by a cavity of a blackbody in the wavelength interval l to l + Dl (Dl denotes an increment of wavelength) can be written in terms of Planck's constant (h), the speed of light (c), the Boltzmann constant (k), and the absolute temperature (T): The wavelength of the emitted radiation is inversely proportional to its frequency, or l = c/n. The value of Planck's constant is found to be 6.6260755 10-34 joule-second. For a blackbody at temperatures up to several hundred degrees, the majority of the radiation is in the infrared region of the electromagnetic spectrum. At increasingly higher temperatures, the total radiated energy increases, and the intensity peak of the emitted spectrum shifts to shorter wavelengths so that a significant portion is radiated as visible light.