Expert: Daniel Mazur - 10/1/2009

Question
Hi Daniel,

I read about stefan's law and black body radiation.
But I don't really understand what does it mean physically when we say that an object "absorbs radiation" , it "reflects a given part of the radiation falling on it" and it "emits radiation".

I know that thermal radiation is nothing but electromagnetic waves which are emitted by the material in question.

Now, when we say that something has absorbed so and so much of radiation, do we mean that it's the constituent atoms or molecules of the substance that have absorbed the energy of the EM waves falling on it?? Or is it the electrons that absorb that energy??
If it is the electrons, why don't they emit that much of radiation back, since excited or accelerating electrons should emit EM waves.
And if it's the entire atom that absorbs the energy of the EM waves falling on it, how does it do this?? I mean what is the mechanism for that ?? is it that the energy of the EM wave suddenly dissappears and then reappears as the increased random motion of the atom??

Also, when we say that the thing "emits" radiation, isn't it just the excited or accelerating electrons emitting EM waves?? And if it is so, it should be the energy of the electron that should decrease. how does it decrease the random motion of the entire atom or molecule, thus reducing the temperature of the body???

And also what does the term "reflection" mean physically??

Basically I'm facing problems understanding, what part of the atom absorbs or emits and how exactly.

Thanks,
Shikhin

Answer
Hi Shikhin,

black body radiation is electromagnetic, because it is considering vacuum as the medium, where the only way of heat (energy) exchange is radiation. Where you have another medium (solid, fluid,...) the majority heat transfer channel is by conduction (atoms vibrating about a stationary position, only energy flows) or convection (energetic atoms/molecules flow, while carrying energy). In the cavity, which people usually consider as a black body, there is no mass that would conduct or convect, so the walls are thermalized by photon exchange only. In any real system the walls can also conduct heat. But as soon as the black body (real or model) is in an equilibrium, there is zero NET flow of heat by conduction OR radiation.

An atom can either reflect or absorb or transmit incoming energy. Transmission means that the radiation does not interact with the matter, so it's not interesting - just something to balance the equations out.

Reflection occurs essentially by an electron absorbing a photon and immediately re-emitting another one at the same energy/frequency, but generally in a different direction. Because the photon energy before and after is the same, this is mathematically equivalent to an elastic collision in a billiard-ball model (with one or two small added tweaks).

Atom absorbing energy usually does it in two different steps: 1) an electron in its orbitals gets excited to a higher energy level allowed to it, 2) the electron (provided it was not altogether kicked out of the atom) looses its energy again from the excited state to its previous one by means of a shower of transitions ("shake-off" is the word) and the energy ends up in the vibrations of the atom and an occasional emission of a photon. Meanwhile the atom exchanges vibrational energy (phonons) with neighboring atoms. The result of a material absorbing some net amount of radiation is an increase in the material's temperature (which is a macroscopic description of the atom's vibrations on average).

The emission of a photon by atoms has nothing to do with accelerating electrons in the classical sense! Emission normally begins with an atom in an excited state, where it could have been pushed by receiving a kick from its neighbor (vibrational energy exchange) or from any particle flying in from afar (gas molecule, a cosmic muon,...), or by absorbing a photon. The non-equilibrium of the atom means that at least one electron is lying at a higher energy level than it would in equilibrium. One of the ways for the atom to react is to emit a photon. Inside the atom the excited electron would drop from the excited level to the equilibrium level (like an instantaneous slow-down), and the energy difference between the two levels will be the energy of the emitted photon. All these processes occur at a different rate, where the rate is wildly dependent on the type and initial thermal state of the atom and the amount of excess energy the atom has before emitting. Once again we describe these by measuring macroscopic properties of materials: absorption cross sections of all kinds, heat conductivity, albedo,...

This should help you in your studies, I think.
Cheers,
Daniel