Blackbody radiation

The term blackbody radiation refers to electromagnetic radiation emitted by a perfectly black object. Such an object is referred to as a blackbody since it absorbs all of the radiation that falls on it and thus appears to be colorless, or black. According to Kirchoff’s law, first stated by Gustav Kirchoff (1824–1887) in 1859, any blackbody must also be a perfect emitter of radiation.

No real object fits the definition of a blackbody since any material reflects some small fraction of the light falling upon it. Soot, carbon black, platinum black, and carborundum are among the real-world materials that come closest to having blackbody properties.

The concept of an idealized blackbody is important in physics. It serves as a standard for heat and temperature measurements just as the wavelength of light emitted by krypton-86 atoms is a standard for measurements of length.

For research purposes, physicists replicate the principle of a blackbody with a device known as a cavity radiator. A cavity radiator is a hollow sphere with a small hole through which radiation can enter and leave. Radiation that enters the hole is reflected continuously within the sphere until it is completely absorbed (as would be the case with a blackbody). It follows, then, that any radiation emitted by the cavity radiator corresponds to the definition of blackbody radiation.

The study of blackbody radiation was of considerable interest to physicists in the late 1800s. Experiments showed that for any given temperature, the intensity (brightness) of blackbody radiation is a maximum for a relatively narrow range of wavelengths, dropping off sharply at shorter and longer wavelengths. A number of attempts were made to use classical electromagnetic theory to derive a mathematical formula that would describe the intensity/wavelength relationship, but all failed for one or another part of the curve. Finally, in 1900, the German physicist Max Planck (1858–1947) solved the problem. By assuming that radiation travels not in continuous waves, but in discrete “packages” (called quanta), Planck was able to derive a formula for the blackbody radiation curve. That formula is as follows:

I = (ħc²/λ⁵)[exp(ħc/λkT – 1]^–1

where ħ is Planck’s constant, k is Boltzmann’s constant, λ is the wavelength of the radiation, and T is temperature in Kelvins.