Kirchhoff's law of radiation (G.R. Kirchhoff):
The emissivity of a body is equivalent to its absorptance at the simialr temperature.
Kirchhoff's laws (G.R. Kirchhoff)
Kirchhoff's first law
An incandescent solid or gas under high pressure will generate a continuous spectrum.
Kirchhoff's second law
A low-density gas will radiate an emission-line spectrum along with an underlying emission continuum.
Kirchhoff's third law
Continuous radiation viewed by a low-density gas will generates an absorption-line spectrum.