Absorption spectroscopy can be used as experimental support of the new derivation of Planck’s Law presented in Mathematical Physics of Blackbody Radiation and Computational Blackbody Radiation and discussed in the recent post Two Proofs of Planck’s Law vs Backradiation.
The experimental setup is illustrated in the above picture showing the recording of two spectra: An absorption line spectrum originating from a continuous spectrum (e.g. a blackbody spectrum) after passage through a cooler gas and an emission line spectrum from the cool gas after being lighted by the source:
The absorption line spectrum is the continuous spectrum with certain frequencies being absorbed by the cool gas which reappear in the emission line spectrum from the cool gas:
- absorption spectrum + emission spectrum = continuous source spectrum.
The absorption line spectrum gives information about the constituents of the gas.
Notice that the gas is cooler than the light source, which means the continuous spectrum reaches beyond the cut-off frequency of the gas. Some of these frequencies resonate with the gas molecules and cause heating of the gas until they are re-emitted with original source amplitude.
If the gas is warmer than the source, then the absorption lines will be missing and the spectrography will not give any information about the gas.
In the case of IR radiation from the Earth looking down from above the atmosphere, the absorption/emission line spectrum is represented by the dip in the spectrum displayed in The Empty Postulate of CO2 alarmism.
Absorption spectrography shows the uni-directional aspect of the interaction between two blackbodies displayed in the new derivation of Planck’s Law with transfer of heat energy from warm to colder, without possibility of backradiation with heat transfer the other way.
This adds to the advantages of the new derivation of Planck’s law as compared to that by Planck: In Planck’s derivation absorption is followed by emission and the case of absorption with emission replaced by intermal heating is not really an option. But this is the case in absorption spectrography which thus is covered by the new proof but not really by the old.
This post was stimulated by the comments given by Doug Cotton on Climate Etc. concerning the Sky Dragon, blackbody radiation and backradiation.