A blackbody is defined as an object that does not reflect or scatter radiation shining upon it, but absorbs and re-emits the radiation completely. A blackbody is a kind of an ideal radiator, which cannot exist in the real world. Yet many objects behave very much as if they were blackbodies.

The radiation of a blackbody depends only on its temperature, being perfectly independent of its shape, material and internal constitution. The wavelength distribution of the radiation follows Planck's law, which is a function of temperature only. The intensity at a frequency v of a blackbody at temperature T is

2hv3

where h = the Planck constant = 6.63 x 10-34 J s , c = the speed of light ~ 3 x 108 ms-1 , k = the Boltzmann constant = 1.38 x 10-23 JK-1 .

By definition of the intensity, the dimension of Bv is Wm-2 Hz-1 sterad-1.

Blackbody radiation can be produced in a closed cavity whose walls absorb all radiation incident upon them (and coming from inside the cavity). The walls and the radiation in the cavity are in equilibrium; both are at the same temperature, and the walls emit all the energy they receive. Since radiation energy is constantly transformed into thermal energy of the atoms of the walls and back to radiation, the blackbody radiation is also called thermal radiation.

The spectrum of a blackbody given by Planck's law (5.12) is continuous. This is true if the size of the radiator is very large compared with the dominant wavelengths. In the case of the cavity, this can be understood by considering the radiation as standing waves trapped in the cavity. The number of different wavelengths is larger, the shorter the wavelengths are compared with the size of the cavity. We already mentioned that spectra of solid bodies are continuous; very often such spectra can be quite well approximated by Planck's law.

We can also write Planck's law as a function of the wavelength. We require that Bv dv = -BA dA. The wavelength decreases with increasing frequency; hence the minus sign. Since v = c/A, we have