I Why is light emitted from an object when it is heated?

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When I think about the structure of an atom and its tightly bound subatomic particles, it is a different regime than the 'world of molecules' and their motion. The early theory of light emission (Bohr, etc) described the emission of light as a phenomenon associated with the transition of an electron from a high energy state to a lower energy state. And the frequency of such light is dependent on the specific energy of the transition. Now when I think of heat, I imagine it to be a jiggling of atoms and collisions of atoms. Typically this is something I associate with a solid although certain liquids and gases can be made to glow but usually not with heat.Rather electricity.So how is it than simply jiggling atoms can send their electrons into high energy states? Certainly it's a question of degree (pun?) Certainly, Enough collision energy can get nuclei to misbehave(fission, fusion).

But then I think of the quantized behavior of the inverse process- - how much light is needed to eject an electron (photoelectric effect? The ultraviolet catastrophe paradox was resolved with the assertion that no matter how much light you shined on an object, you could not get electrons to leave those atoms unless the light had a high enough frequency (energy per photon). But I guess I'm asking why light emission caused by heat is not quantized in the same way as electron emission caused by light? Is not the same quantum rule in play? You might say 'how do you quantize heat' in the same way as light' - I do not have the facility with quantum mechanics to construct a reasonable model.
 

DrClaude

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Blackbody emission is definitely quantised, as the work of Planck on the blackbody spectrum is the major spark that got all that quantum stuff going.

I think that your questioning comes from the fact that you are thinking of solids as made up of atoms that retain their atomic character. That's not the case. Solids (for the most part) do not have discretely quantised energy levels but continuous energy bands. With enough pressure, even gases can give quite broad spectra.
 
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Blackbody emission is definitely quantised, as the work of Planck on the blackbody spectrum is the major spark that got all that quantum stuff going.

I think that your questioning comes from the fact that you are thinking of solids as made up of atoms that retain their atomic character. That's not the case. Solids (for the most part) do not have discretely quantised energy levels but continuous energy bands. With enough pressure, even gases can give quite broad spectra.
thank you for your reply. indeed light is quantized but that's not what i am asking. I am asking why isnt there a 'quantum description of heat'. The stupidity of this question is based on my making an analogy to the photoelectric effect. The observation that the ejection of electrons requires a certain energy per photon. Raising the frequency of the photon does not make more electrons come out though.

I perceive the process of an atomic collision being the incipient event to a higher energy state just as a photon collision is in the photoelectric effect. I called it the 'inverse' because it is atomic collison (electrons actually in the outer atomic shell) that produces higher energy states leading to light. Whereas in the photoelectric effect, it is photons colliding with atoms ejecting electrons or just causing heat.

In the photoelectric effect we characterize photon energy as a function of frequency. In atomic collisions, atomic collision energy is not quantized. Sounds stupid, after all kinetic energy is a function of velocity, a continuous variable. But if the analogy were to hold, I would predict light emission not to occur until a certain energy(temperature). And beyond that energy, the light emission should not change. But that is not what we see. As temp rises, the color changes. If light emission is quantized, the the spectra should not change.or maybe i am confused as to why the color changes as the object gets hotter.
 

ZapperZ

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thank you for your reply. indeed light is quantized but that's not what i am asking. I am asking why isnt there a 'quantum description of heat'.
First of all, who said there isn't?

But you also need to understand the problem here. "Heat" is actually a rather vague terminology, because it occurs in several different "media". It can be in solids (lattice vibrations, which are phonons, and thus, are already "quantized"), it can be molecular/atomic motion in gasses, and it can be simply a quantity of energy in EM radiation (which is already quantized in terms of photons).

This is why there is no single "quantum" picture or description of heat. It depends on where it occurs!

Zz.
 
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First of all, who said there isn't?

But you also need to understand the problem here. "Heat" is actually a rather vague terminology, because it occurs in several different "media". It can be in solids (lattice vibrations, which are phonons, and thus, are already "quantized"), it can be molecular/atomic motion in gasses, and it can be simply a quantity of energy in EM radiation (which is already quantized in terms of photons).

This is why there is no single "quantum" picture or description of heat. It depends on where it occurs!

Zz.
Sir you speak in riddles. Please elaborate on your last six words.
 

ZapperZ

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Sir you speak in riddles. Please elaborate on your last six words.
In the post, I described to you THREE situations of what one considers as "heat" (lattice vibrations, molecular/atomic movement, and EM energy). These correspond to the "where", i.e. solids, liquid/gas, and vacuum.

So which part of this did you not understand?

Zz.
 
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ok. let's take the example of atomic movement since that is what i described in post#3 above. You use a flame to heat iron metal. The color starts out red and then becomes white as the temp rises. http://www.giangrandi.ch/optics/blackbody/blackbody.shtml
So the frequency of light emitted when I agitate atoms changes as the energy increases. That means that as I jiggle atoms more vigorously, somehow the electrons get kicked up to higher energies and then the electrons jump back down to their base energy with the release of a photon.

Since light emission is quantized (https://en.wikipedia.org/wiki/Photoelectric_effect), only a specific frequency should appear. But that is not what we see when we heat metal. We see the color change with increasing temperature. Furthermore, blackbody radiation is not dependent on the material. The color of the emitted light is dependent only on the temperature. Yet if light is associated with electron state transitions, then different materials (with different electronic structures) should have electrons that change state at unique energies. So the color of one thing heated to 2000 degrees should be different from another thing at 2000 degrees. I would predict this because at that temperature one material might have electrons that make a big jump and another material might have electrons that make a little jump.
 

Drakkith

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So the frequency of light emitted when I agitate atoms changes as the energy increases. That means that as I jiggle atoms more vigorously, somehow the electrons get kicked up to higher energies and then the electrons jump back down to their base energy with the release of a photon.
Electronic transitions are only one possible mechanism for systems of charged particles to emit radiation. Other mechanisms include charge-acceleration and dipole oscillation, neither of which involve electrons changing atomic or molecular energy levels. Basically the atoms/molecules can vibrate and act like a dipole antenna, or the repulsive and attractive forces can accelerate charges, both of which emit radiation. The vast majority of thermal radiation emitted from an object is not from electronic transitions.
 
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Drakkith: thank you. I had no idea there are so many mechanisms. That explains a lot for me. Where can I read more about this? Physics texts are usually abstract and focus on equations rather than the process of discovery. I’d like to know how these mechanisms were identified, which energy regimes they dominate, etc
 

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Henryk

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That means that as I jiggle atoms more vigorously, somehow the electrons get kicked up to higher energies and then the electrons jump back down to their base energy with the release of a photon.
This is correct. That's why the color changes as we go up with temperature. We have more electrons thermally excited to higher energy levels and whey they fall back they emit photons or higher energy, i.e. more bluish.
Since light emission is quantized (https://en.wikipedia.org/wiki/Photoelectric_effect), only a specific frequency should appear. But that is not what we see when we heat metal. We see the color change with increasing temperature. Furthermore, blackbody radiation is not dependent on the material
That is correct too. Different material will glow differently. Once, I was playing with a piece of magnesium oxide crystal. It is transparent. I put it on a fire brick, heated it with a propane torch. The brick underneath started glowing red but there was absolutely no glow from the magnesium oxide. Why? because it is transparent. It does not have any electronic transition corresponding to photons of energy in the visible range (that's why it is transparent) and it cannot emit any photon in that range either. So, there you go, the fire brick (light colour but not transparent) was glowing bright red but the transparent crystal on top of it and heated to a somewhat higher temperature did not glow at all.
Most metals are gray to a different extent. That means they will absorb light of a wide frequency range in the visible spectrum and, therefore, can emit light in the same spectral range.
As for black body. There is no material that is 100 % black. The laboratory implementation of a black body is actually a cavity with a small hole leading to the outside world.
 

Drakkith

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This is correct. That's why the color changes as we go up with temperature. We have more electrons thermally excited to higher energy levels and whey they fall back they emit photons or higher energy, i.e. more bluish.
No, that is not correct. See my post.

That is correct too. Different material will glow differently. Once, I was playing with a piece of magnesium oxide crystal. It is transparent. I put it on a fire brick, heated it with a propane torch. The brick underneath started glowing red but there was absolutely no glow from the magnesium oxide. Why? because it is transparent. It does not have any electronic transition corresponding to photons of energy in the visible range (that's why it is transparent) and it cannot emit any photon in that range either.
That is also incorrect. This red-hot glass is a perfect example of how a transparent material can glow:

aPHo9.jpg
 

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Henryk

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Drakkith,
Actually, I realized that my statement is not quite correct. The correct version would be the radiation is emitted when there is a transition between quantum states, no necessarily electronic. Vibrational transition are the major contributor to EM radiation and emission in infrared.
I still stand y assertion that it is transition between the quantum states that is producing radiation. You say
Other mechanisms include charge-acceleration and dipole oscillation, neither of which involve electrons changing atomic or molecular energy levels
Yes, in classical picture, acceleration of a charge produces EM radiation. But if you look at it from the quantum point of view, a stationary state for an electron in free space is the one of the constant momentum. If you accelerated charge, you change its momentum and that is a transition between different quantum states, isn't it?
You don't also agree with my second statement regarding glowing of hot bodies.
This red-hot glass is a perfect example of how a transparent material can glow:
Well, ordinary glass is not that transparent, in fact, a few mm of glass will absorb a few percent of light in the visible range (take a close look a piece of glass, preferably looking into an edge of it and you will definitely see a greenish tint). Therefore, when you heat a piece of glass up, it will glow proportionally to the absorption coefficient. If you ever worked with fused silica (quartz) you find it much more transparent and I can tell you from my own practice. Heated with a propane torch, a piece of glass tube will glow, but the glow of fused silica is not visible under ordinary light. The piece of magnesium oxide did not glow at all. It was just too transparent.
 

Drakkith

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If you accelerated charge, you change its momentum and that is a transition between different quantum states, isn't it?
I'm not an expert on QM, but that sounds correct to me.

Well, ordinary glass is not that transparent, in fact, a few mm of glass will absorb a few percent of light in the visible range (take a close look a piece of glass, preferably looking into an edge of it and you will definitely see a greenish tint). Therefore, when you heat a piece of glass up, it will glow proportionally to the absorption coefficient. If you ever worked with fused silica (quartz) you find it much more transparent and I can tell you from my own practice. Heated with a propane torch, a piece of glass tube will glow, but the glow of fused silica is not visible under ordinary light. The piece of magnesium oxide did not glow at all. It was just too transparent.
This is the first I've heard about such an effect. Do you have any references that would elaborate on this?
 

ZapperZ

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In a synchrotron light source facility, light is produced by bunches of electrons being jiggled by insertion devices such as a wiggler or undulator. This is a similar process for a FEL.

There are no “transition”here.

Zz.
 

Henryk

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Drakkith
I don't have any reference on it. It is just observation I made while working at a lab.
There are, of course, various grades of glasses. some have more, some have less tint. In any case, none is 100 %transparent. Fused silica glass is much clearer, has much less tint. I used to seal specimen of sensitive material in glass tubes by evacuating the tube and sealing it off with a torch. One day, it picked a fused silica tube and tried to seal a specimen in it. I heated it up and two things didn't happen: it didn't glow, and it didn't soften and sealed. (The second was because fused silica softens at twice the temperature for the glass and my torch wouldn't heat it that high).
 

Henryk

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Zapper
In a synchrotron light source facility, light is produced by bunches of electrons being jiggled by insertion devices such as a wiggler or undulator. This is a similar process for a FEL.

There are no “transition”here.
There IS a transition, as a matter of fact an infinite number of transition between the states of different momenta as the moving electrons oscillate in the magnetic field.
 

ZapperZ

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Zapper

There IS a transition, as a matter of fact an infinite number of transition between the states of different momenta as the moving electrons oscillate in the magnetic field.
But there are no discrete momentum states. It is a continuous change in momentum, which is what classical physics is.

You need to show how bunches of electron oscillating with a particular frequency can produce a distinct frequency of light while undergoing all these "infinite transition". How do these infinite transitions produce such a distinct fundamental frequency of light?

Zz.
 
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DrClaude

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Since light emission is quantized (https://en.wikipedia.org/wiki/Photoelectric_effect), only a specific frequency should appear.
This will almost never be the case. In a gas, you will first of all see Doppler broadening, due to the spread in velocity of the molecules in the gas, and at higher densities, collisional broadening. And as I said above, in the case of condensed matter, you can't see it as a collection of individual atoms, but rather collective effects abound, and the emission spectrum gets quite broad.
 

Henryk

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But there are no discrete momentum states. It is a continuous change in momentum, which is what classical physics is.
Discrete energy states happen if and only if a particle is localized. A textbook case is a particle in a finite potential well. When energy is negative, the energy states are discrete. For positive energy, the energy spectrum is continuous but not the same as for a particle in free space.
 

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