Where does thermal radiation come from?

[Entanglement]

Where does thermal radiation come from? vibration of the molecules or the excitement of electrons ?

 

[Drakkith]

Both.There are many more energy states available for electrons in bulk materials than in single atoms and small molecules, so you see broad ranges of wavelengths emitted from these states in addition to those emitted due to vibrations.

 

[Entanglement]

Even when we talk about blackbody radiation, the meant radiation is due to vibration and electron excitement ??

 

[DrClaude]

It depends on temperature, but all degrees of freedom (except translational) contribute: rotational, vibrational, and electronic.

 

[Entanglement]

It depends on temperature, but all degrees of freedom (except translational) contribute: rotational, vibrational, and electronic.

What is translational and rotational ??

 

[DrClaude]

What is translational and rotational ??

When a molecule moves and when it turns.

 

[Entanglement]

When a molecule moves and when it turns.

So at all temperatures, radiation comes from vibrational, rotational motion and electron excitement,
And in case of gases we can say translational too.

 

[DrClaude]

So at all temperatures, radiation comes from vibrational, rotational motion and electron excitement,

No, it depends on temperature. For instance, at room temperature CO₂ will emit radiation due to rotational and vibrational excitation, but there will be basically no electronic excitation.

And in case of gases we can say translational too.

No. There is never emission due to translational motion (it doesn't couple to the electromagnetic vacuum).

 

[Entanglement]

Cool that's great, there is one more thing that I can't understand. What's special about blackbody radiation that makes it different from the general thermal radiation ?

 

[DrClaude]

Cool that's great, there is one more thing that I can't understand. What's special about blackbody radiation that makes it different from the general thermal radiation ?

A blackbody can absorb radiation of any frequency and, consequently, emit at all frequencies. That's what makes it special! It is an idealization of reality: for instance, the Sun is not an actual blackbody, in part because the solar atmosphere absorbs some of the radiation (that's how we figure out what elements compose stars). The Earth's atmosphere is sometimes also modeled as a blackbody, although it obviously doesn't absorb in the visible region.

At the other extreme from blackbodies are single atoms or molecules, which can absorb/emit only discrete frequencies.

 

[Entanglement]

A blackbody can absorb radiation of any frequency and, consequently, emit at all frequencies. That's what makes it special!

Can you please explain this part, why do atoms absorb radiation and why do they re-emit it ??

 

[Drakkith]

Can you please explain this part, why do atoms absorb radiation and why do they re-emit it ??

Electromagnetic radiation is created by the acceleration of electrical charges. Atoms are made up of electrically charged particles and in a large object these charged particles are constantly moving about and are being accelerated in different directions by interacting with each other. This constant random motion generates a broad spectrum of radiation at different wavelengths.

EM Radiation is made up of electromagnetic waves. These waves cause electrically charged particles to accelerate, transferring energy from the wave to the particles. Since atoms are made up of charged particles, they interact with EM waves and absorb them.

 

[Entanglement]

Electromagnetic radiation is created by the acceleration of electrical charges. Atoms are made up of electrically charged particles and in a large object these charged particles are constantly moving about and are being accelerated in different directions by interacting with each other. This constant random motion generates a broad spectrum of radiation at different wavelengths.
EM Radiation is made up of electromagnetic waves. These waves cause electrically charged particles to accelerate, transferring energy from the wave to the particles. Since atoms are made up of charged particles, they interact with EM waves and absorb them.

We we say that the atom absorbs EM waves we actually mean the electrons??

 

[Drakkith]

We we say that the atom absorbs EM waves we actually mean the electrons??

I believe the entire atom (or molecule) can absorb energy and split it between different energy states, including the electronic, rotational, and vibrational states. I'm not certain though.

 

[Entanglement]

I believe the entire atom (or molecule) can absorb energy and split it between different energy states, including the electronic, rotational, and vibrational states. I'm not certain though.

At the selected frequencies will be according what ??

 

[DrClaude]

I will start with the simple case of atoms. It is the combination of the nucleus and the electron that can absorb a photon (a free electron will not exchaneg energy with an electromagnetic field), but since the nucleus is much heavier than the electron, people often talk in terms of electrons only, using a picture where the nucleus is fixed and electrons are found in orbitals (the quantum mechanical equivalent to planetary orbits). These orbitals are quantized, such that electrons can only be found in discrete energy levels. Consequently, an atom can only absorb or emit photons of an energy that corresponds exactly to the difference in energy between two levels. This is why an atom will only absorb or emit at very precise frequencies; see for example what this looks like for hydrogen.

When considering a molecule, things get more complicated as you have to consider also the motion of the nuclei. In that case, you do not have the motion of opposite charges, as in the case of nuclei and electrons, but molecular bonds are often polar: the charge is not spread out evenly between two atoms and that creates a dipole. That dipole can interact with an electromagnetic field, and again you have absorption/emission of photons. The corresponding motion inside the molecule is vibration, and this is also quantized, so again only discrete frequencies appear in the spectrum. Rotation is similar, corresponding to the interaction of a rotating dipole with the electromagnetic field.

Things get more complicated when you consider solids. First, the presence of other molecules will shift the energy of the vibrational levels, and you can end up with energy bands. In the case of metals, you also have the interaction of the electromagnetic field with the conduction electrons.

It is hard to get into all the details in a forum. You will have to find a good book and read up on the subject.

 

[Entanglement]

I will start with the simple case of atoms. It is the combination of the nucleus and the electron that can absorb a photon (a free electron will not exchaneg energy with an electromagnetic field), but since the nucleus is much heavier than the electron, people often talk in terms of electrons only, using a picture where the nucleus is fixed and electrons are found in orbitals (the quantum mechanical equivalent to planetary orbits). These orbitals are quantized, such that electrons can only be found in discrete energy levels. Consequently, an atom can only absorb or emit photons of an energy that corresponds exactly to the difference in energy between two levels. This is why an atom will only absorb or emit at very precise frequencies; see for example what this looks like for hydrogen.

When considering a molecule, things get more complicated as you have to consider also the motion of the nuclei. In that case, you do not have the motion of opposite charges, as in the case of nuclei and electrons, but molecular bonds are often polar: the charge is not spread out evenly between two atoms and that creates a dipole. That dipole can interact with an electromagnetic field, and again you have absorption/emission of photons. The corresponding motion inside the molecule is vibration, and this is also quantized, so again only discrete frequencies appear in the spectrum. Rotation is similar, corresponding to the interaction of a rotating dipole with the electromagnetic field.

Things get more complicated when you consider solids. First, the presence of other molecules will shift the energy of the vibrational levels, and you can end up with energy bands. In the case of metals, you also have the interaction of the electromagnetic field with the conduction electrons.

It is hard to get into all the details in a forum. You will have to find a good book and read up on the subject.

Thanks a lot your are awesome !

So that what makes every object to have a certain color, it absorbs a part of the white light then it reflects the rest. The reflected light is the color that we observe and see !

I really appreciate your great effort and patience.

I want to know when the matter absorbs EM waves with a certain frequency can can it re-emit it ?? For electrons I know that the wave or more specifically the photon is re-emitted after 10 to the power -8 seconds, but what about the EM waves that is absorbed by the atom itself for rotational and vibrational motion, when is it re-emitted?

 

[DrClaude]

I want to know when the matter absorbs EM waves with a certain frequency can can it re-emit it ?? For electrons I know that the wave or more specifically the photon is re-emitted after 10 to the power -8 seconds, but what about the EM waves that is absorbed by the atom itself for rotational and vibrational motion, when is it re-emitted?

Again, there is no simple answer here. Different excited states have different lifetimes. Some are very short lived, while others can live on for hours (look up phosphorescence). What complicates things is that if an atom or molecule is excited from state A to state Z, it does not necessarily decay back from Z to A, but can go through intermediate states.

That said, generally you can take that electronically excited states are shorter lived than vibrationally excited states, which are shorter lived than rotationally excited states.

 

[Entanglement]

Again, there is no simple answer here. Different excited states have different lifetimes. Some are very short lived, while others can live on for hours (look up phosphorescence). What complicates things is that if an atom or molecule is excited from state A to state Z, it does not necessarily decay back from Z to A, but can go through intermediate states.
That said, generally you can take that electronically excited states are shorter lived than vibrationally excited states, which are shorter lived than rotationally excited states.

If re-emission occurs in all cases then all objects should have the white color since any matter reflects the Unabsorbed EM waves then re-emits what it absorbed.

 

[DrClaude]

If re-emission occurs in all cases then all objects should have the white color since any matter reflects the Unabsorbed EM waves then re-emits what it absorbed.

If A→Z is not followed by Z→A, but by emission from an intermediate state, Q → A, then the light emitted will not be of the same frequency. Going from Z to Q might be through emission, but again not the same frequency as the absorbed photon, or by (radiationless) internal conversion.

 

[Entanglement]

If A→Z is not followed by Z→A, but by emission from an intermediate state, Q → A, then the light emitted will not be of the same frequency. Going from Z to Q might be through emission, but again not the same frequency as the absorbed photon, or by (radiationless) internal conversion.

That's quite clear, thanks a lot.

So what's special about black body radiation is that it absorbs all frequencies when it's exposed to all the frequencies of the spectrum, consequently when it's heated it will emit all these frequencies. But that's just theoretical, there isn't an ideal blackbody. Isn't that right ?

 

[Drakkith]

That's quite clear, thanks a lot.

So what's special about black body radiation is that it absorbs all frequencies when it's exposed to all the frequencies of the spectrum, consequently when it's heated it will emit all these frequencies. But that's just theoretical, there isn't an ideal blackbody. Isn't that right ?

A perfect black body is indeed completely theoretical and does not exist. Most solids are approximate black bodies to some degree and will deviate from the perfect black body spectrum some amount.

 

[Entanglement]

A perfect black body is indeed completely theoretical and does not exist. Most solids are approximate black bodies to some degree and will deviate from the perfect black body spectrum some amount.

what did make Planck assume a deviation from reality and theorize the blackbody??

Ideal gases was theorized to facilitate the study of gases and to idealize the gas laws and to put an equation that fits all the gases "PV=nRT"

I mean that the main reason for assuming perfection or idealization is to facilitate the reality.

What was the reason of putting the assumption of blackbody ?

 

[DrClaude]

It's part of the history of the development of thermodynamics. For starters, see
http://en.wikipedia.org/wiki/Black-body_radiation#History