# Heating by radiation: how the atoms' speed is increased?

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If you heat something by radiation, its temperature increases, which means there is an increase of the average velocity of the atoms the "something" is made of.

Another thing which happens when radiation hit the object, is that its atoms can "absorb" photons, as electrons go to higher energy states, and similary the nuclei go into excited states.

So my question is basically : how the speed of the atoms increase when they receive photons ? I mean, I suppose the gained kinetic energy is not the same thing as the energy gained by going into excited states (or is it ?). Is there then a mechanism by which the ernegy of a photon is somehow converted into kinetic energy for the atom ?

For heat conduction (and convection), I can understand that collisions between atoms of the hot and cold bodies transfer kinetic energy. I'm curious of how this works with radiation.

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OmCheeto

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ZapperZ
Staff Emeritus
Radiation and photons are characterized by, among other things, wavelengths/frequencies. Matter interact and react differently with EM radiation of different frequencies. Not ALL EM radiation frequencies can cause atomic excitation. Not all EM radiation frequencies can call photoelectric effect. And not all EM radiation frequencies can eat up matter.

Most matter some some sort of a natural frequencies, be it due to the molecular bonds that make up the material, or the natural mode of vibration of the lattice ions that form the solid. And in our understanding of resonance phenomenon, we know that if an incoming radiation has a frequency that matches such natural frequencies of the material, then it may absorb such radiation to cause the material to heat up. That's how microwave ovens work.

At at these wavelengths, the energy is too low to cause atomic transition or electron emission. This is why one must not only understand the mechanisms, but one must also have ball-park idea of the quantitative aspect of these mechanisms and phenomena, i.e. what value of energy is needed for such-and-such to occur with significant probability.

Zz.

Thanks for the precisions @ZapperZ. This doesn't explain how atoms can gain kinetic energy from photons though, if they don't absorb them.

I just thought of something : could it be by successive absoprtion and emission ? Which is I think nothing more than reflection ? So that the gained momentum of the atom would correspond to the reversed momentum of "reflected" photons. Not sure this is so simple though. Also it'd need for the reflected photon to be of a lower frequency (##p = h/\lambda = hf/c## ?), so that momentum can be conserved.

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ZapperZ
Staff Emeritus
Thanks for the precisions @ZapperZ. This doesn't explain how atoms can gain kinetic energy from photons though, if they don't absorb them.
But molecular and lattice vibrations do NOT require individual atoms to absorb photons!

Unfortunately, you have the impression that energy levels only exist for atoms. This is not true. There are vibrational, rotational, etc energy levels as well! And in a solid, there are BANDS of states that form conduction bands, valence bands, etc. Atomic levels are not the only ones in existence.

I just thought of something : could it be by successive absoprtion and emission ? Which is I think nothing more than reflection ? So that the gained momentum of the atom would correspond to the reversed momentum of "reflected" photons.
You need to get your mind out of the narrow view of individual atoms having a role when they form, say, a solid. Once atoms get together to form a solid, they lose a lot of their individuality. It is why we have solid state physics/condensed matter physics as a discipline, and that they are separate from "atomic" physics. For example, take a bunch of carbon atoms, and arrange them one way, and you get graphite. Yet, take the SAME carbon atoms, and arrange them in a different way, and you get diamond! Two very different material, yet, they are both made of the SAME type of atom!

This clearly shows that it is not just type of atoms that matters, but also how they are put together! It shows that the COLLECTIVE properties of the atoms now matters a great deal, and this collective properties are ABSENT when you simply look at individual atoms. In the words of physicist Phil Anderson, "More Is Different!".

When a solid or molecule absorbs light and causes it to vibrate, that vibrational energy be lost as heat, and it may be conducted away. The absorbed radiation doesn't get remitted (did your microwaved food re-emit microwave radiation when it cooled down?). There are more ways for the energy to be dissipated than just re-radiating.

Zz.

OmCheeto
But molecular and lattice vibrations do NOT require individual atoms to absorb photons!
All right, but if the material does not absorb photons, by what mechanism related to the radiation it receives its temperature increases ? Even at the collective level, there must be something happening.

When a solid or molecule absorbs light and causes it to vibrate, that vibrational energy be lost as heat, and it may be conducted away.
Now you're actually saying that photon absorption causes atoms to gain KE. :D

The absorbed radiation doesn't get remitted (did your microwaved food re-emit microwave radiation when it cooled down?).
Well, it does dot re-emit microwaves, but it does emit photons (infrared I think ?). And the amount of radiation it emits is proportionnal to the fourth power of its temperature (Stefan-Boltzmann law, well ok my food isn't a black body, but still, the hotter it is, the more it radiates). Its temperature depends of how much I heated it with the microwave oven, so yeah it actually does re-emitt part of the energy I put in.

ZapperZ
Staff Emeritus
All right, but if the material does not absorb photons, by what mechanism related to the radiation it receives its temperature increases ? Even at the collective level, there must be something happening.
I don't understand. Where did I say that the material "does not absorb photons"?

What I stated is that ATOMIC TRANSITION is NOT the only way that EM radiation can be absorbed! You were focused simply on electrons in atoms being excited to a higher energy level. I pointed out that there are OTHER mechanism for photons to be absorbed!

Please re-read what I wrote. Otherwise, there is no point in my continuing this discussion.

Zz.

I don't understand. Where did I say that the material "does not absorb photons"?

What I stated is that ATOMIC TRANSITION is NOT the only way that EM radiation can be absorbed! You were focused simply on electrons in atoms being excited to a higher energy level. I pointed out that there are OTHER mechanism for photons to be absorbed!
I meant you said that KE can be gained without photon absorption :

But molecular and lattice vibrations do NOT require individual atoms to absorb photons!
When I read that I think "oh ok so the internal KE gained under radiation exposure does not comes from photon absorption.". That's perfectly fine with me. What is the mechanism responsible then ?

But right after you write :

When a solid or molecule absorbs light and causes it to vibrate
Here you actually say that the internal KE gained can come from photon absorption.

Again, my question is :

So my question is basically : how the speed of the atoms increase when they receive photons ?
If it is due to photon aborption, then question answered. If it is not, then : what is the mechanism causing the solid to gain internal KE under radiation exposure ?

I appreciate the fact that you're trying to help me with my question BTW ! Don't see any offense if I don't get what you mean.

ZapperZ
Staff Emeritus
Please note that when I said:

ZapperZ said:
But molecular and lattice vibrations do NOT require individual atoms to absorb photons!
I meant that the photon absorption is NOT done by INDIVIDUAL ATOMS, but rather by the entire molecule or solid! In other words, photon absorption still happens, but NOT by INDIVIDUAL ATOMS, i.e. not due to atomic transition!

If this is what you read and misunderstood, please reset. Otherwise, you are asking me to explain what I never said!

Zz.

I meant that the photon absorption is NOT done by INDIVIDUAL ATOMS, but rather by the entire molecule or solid! In other words, photon absorption still happens, but NOT by INDIVIDUAL ATOMS, i.e. not due to atomic transition!

If this is what you read and misunderstood, please reset. Otherwise, you are asking me to explain what I never said!
That is indeed what I misunderstood, thank you for the clarification. I must say it seems really weird to me that photon absorption is done by the molecule but not by the individual atoms, since the molecule is made of atoms. But anyway, ok.

Back to my question, is photon absorption (by the molecule) the mechanism by which said molecule gains KE when it is exposed to EM radiation ?

DrClaude
Mentor
Back to my question, is photon absorption (by the molecule) the mechanism by which said molecule gains KE when it is exposed to EM radiation ?
Yes. Heating will occur when de-excitation happens not by emission of a photon, but by non-radiative processes, such as internal conversion (e.g., electronic excitation goes into vibrational excitation) or collisions.

DoobleD
Yes. Heating will occur when de-excitation happens not by emission of a photon, but by non-radiative processes, such as internal conversion (e.g., electronic excitation goes into vibrational excitation) or collisions.
Awesome, thanks. What would be the field of study for those phenomenon ? Nuclear physics maybe ?

OmCheeto
Gold Member
Awesome, thanks. What would be the field of study for those phenomenon ? Nuclear physics maybe ?
In post #4, ZapperZ mentioned:

It is why we have solid state physics/condensed matter physics as a discipline, and that they are separate from "atomic" physics
Having no knowledge of such things, I searched PF for more information, and ran across the following thread:

Hurkyl and marcus kind of explained my confusion on the subject:

Hurkyl said; "The problem, I think, is that the only way to "create" light that is explained in introductory texts is emission -- the mechanism of black body radiation is not described at all. (At least I don't remember it from my freshman physics courses)"

to which marcus responded; "hurkyl you have put yr finger on the main pedagogical point!
they should tell you very early how hot objects actually manage to glow
(with a continuous spectrum planck black body lopsided mound glow)

but they mostly DONT tell you
so people like Smith are going around trying to apply the only model they know for producing light-----namely an electron jumps down one notch---and that is not how most light is produced!"

Why does no one tell us these things..........
I guess I'm going to have to figure out what "condensed matter physics" is all about now.

DoobleD
In post #4, ZapperZ mentioned:
Oh right, didn't see that, thank you.

Concerning the mechanisms for emission of light, that's a different problem. But it's quite true that when learning about BB radiation I was told about stuff like Planck's law and Stefan-Boltzmann's law, but never about how the radiation actually happens. I've read on internet that it's simply due to the acceleration of the vibrating charges. I also tend to forget about photons emitted due to nuclear reactions (gamma and X rays mainly I think).

DrClaude
Mentor
What would be the field of study for those phenomenon ? Nuclear physics maybe ?
No, it has nothing to do with nuclear physics. This is molecular physics. Most of these topics are usually covered as part of molecular spectroscopy. You will also find some info in textbooks on physical chemistry, such as Atkins.

DoobleD