- 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.