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How can any physical object absorb and emit white light when all objects are made from atoms and molecules, each of which has its own absorption/emissions spectrum that acts as it's signature “chemical fingerprint”? Showing a continuous spectrum seems contradictory to the concept of an object being made of atoms. What am I not understanding here?

In fact, the only time I can see that a blackbody could exist is when the object is so hot that it becomes plasma with free electrons, where the electrons can acquire a continuous range of energies and therefore emit a continuous spectrum.

In a related idea, can atoms/molecules absorb or emit light in any other way than electrons being bumped up or falling down from their known quantized energy levels? Maybe I am not taking this into consideration?

What am I missing in my understanding?

Roger V.
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LouL
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2 Answers2

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First, black body radiation is an idealisation.

Further, a collection of atoms of the same type radiate some frequencies more than other frequencies, but they also absorb these frequencies more than other frequencies. When the free path of this radiation is much smaller than the size of the collection of atoms, then the preference for these frequencies in both radiating and absorbing cancel each other.

Fred. Zwarts
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What is missing is the higher order processes/effects. When talking about "chemical fingerprint" we mean the most intensive absorption/emission lines. In reality there are many processes, like Raman scattering, two-photon absorption and emission, etc., which couple radiative modes of different energies. The energy is thus pumped between the modes till equilibrium is established between them (i.e., microscopic balance between the forward and the reverse process.) Note that statistical physics always comes with a caveat that we wait a sufficiently long time for thermodynamic equilibrium to establish. It is only when the sufficiently long turns out longer than the duration of the experiment or the lifetime of the universe or another important scale that we speak of ergodicity breaking, etc. - this situations happen all the time, but typically not for gases (definite polarization of magnets is the most obvious example.)

Note also that even in a non-plasma gas the spectrum lines have natural linewidth and are also Doppler-shifted, depending on the direction where the atom/molecule flies (this is used with a great effect in laser cooling.)

On the other hand, metals are indeed very close to plasma - as the state of their electrons is even termed electron plasma, and these electrons do undergo effects like Bremsstrahlung.

Related
How is light emitted by an incandescent lamp?
Difference in thermal radiation between condensed matter and gases
How does radiation become black-body radiation?
Understanding thermal radiation in a conductor, gas and insulator
How do photons reach thermal equilibrium with the walls of the blackbody cavity?
Quantum and non-equilibrium properties of the radiation, emitted by "thermal light sources"

Roger V.
  • 68,984