- Almost everything emits electromagnetic radiation, of
most wavelengths, all the time.
- Warmer things emit more radiation than cooler things do.
(Hence, the hotter a thing gets, the more radiation it emits.)
- Warmer things emit most of their radiation at
shorter wavelengths; cooler things emit most of
their radiation at longer wavelengths.
- The previous three laws apply most strictly to blackbody
radiators—objects that can absorb any wavelength of radiation
completely and emit any wavelength at the maximum rate theoretically
possible (which for any particular wavelength depends only on the object's temperature). Examples of blackbodies (or nearly so) include the sun and
the Earth as a whole (atmosphere and surface combined).
However, some objects will absorb only some wavelengths and not
others—such objects are called selective absorbers. If a
selective absorber absorbs a particular wavelength of radiation well,
then it will emit that wavelength at the maximum rate theoretically
possible. If a selective absorber doesn't absorb a particular
wavelength well, it won't emit that wavelength, either.