Kirchhoff's Three Laws of Spectroscopy

[omoplata]

Kirchhoff's Three Laws of Spectroscopy are as follows.

1. An incandescent solid, liquid, or gas under high pressure, emits a continuous spectrum.
2. A hot gas under low pressure emits a "bright-line" or emission-line spectrum.
3. A continuous spectrum source viewed through a cool, low-density gas produces an absorption-line spectrum.

http://en.wikipedia.org/wiki/Spectrum_analysis" is the Wikipedia article I took that from.

How does an incandescent solid, liquid or gas under high pressure NOT have an emission line spectrum? Doesn't each atom this material is made up of emit emission lines as an electron transition happens from a higher energy level to a lower energy level? And and atom with an electron at a lower energy level can absorb a photon of correct frequency and transition to a higher energy level, right? Do these two processes cancel each other out?

Let's take a sample of an incandescent solid. I can understand how the emission photons from atoms at the middle of the sample being absorbed as they travel outwards. But if an atom at the outer edge of the sample emits a photon outwards, that doesn't get absorbed, right? So, shouldn't there be at least very faint emission lines?

And I guess even photons emitted from the center of a gas clump that is diffused can travel out through the clump without hitting other atoms because there is enough spatial separation between the atoms?

And how is light that does NOT have the frequency of an emission line get emitted? Maybe a derivation of black body radiation will help me understand this, but is there a simple qualitative explanation?

Thanks.

 

[Twigg]

How does an incandescent solid, liquid or gas under high pressure NOT have an emission line spectrum? Doesn't each atom this material is made up of emit emission lines as an electron transition happens from a higher energy level to a lower energy level?

The relevant effect here is pressure broadening. At high pressure, the atoms/molecules get squeezed very close together. At these short distances, the atoms/molecules start to have strong interactions. These interactions end up shifting the energies/frequencies of the characteristic emission lines. When you average the effect of these interactions over time, you end up seeing a continuous spectrum which is the result of the emission lines being smeared out by intermolecular interactions. If you were to do spectroscopy at a very fast timescale on a single molecule in the high pressure gas, you would still observe discrete emission lines. The critical timescale here is the time it takes for two atoms/molecules to collide and move apart (the elastic collision rate), and this depends on the density of the gas.