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Do just electrons emit photons?

  1. Aug 17, 2015 #1
    Do just electrons emit photons/radiation. Or do atoms and molecules emit photons as well? Just can't get a clear answer on this.

    And if atoms and/or molecules also emit photons, can you please explain what causes them to?
     
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  3. Aug 17, 2015 #2

    Bystander

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  4. Aug 17, 2015 #3
    Good information, but what I'm asking about is not covered there. I need a specific answer regarding whether molecules and atoms, themselves, emit photons. I know electrons emit photons, but do molecules emit them as well? And speaking of blackbody radiation, is that emitted from just electrons or from both electrons and molecules?
     
  5. Aug 17, 2015 #4

    mathman

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  6. Aug 17, 2015 #5

    Bystander

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    Re-read the part of the article on "cavity radiation."
     
  7. Aug 17, 2015 #6

    ZapperZ

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    If I take a bunch of protons and jiggle it up and down, I can generate light.

    ANY accelerated charge will create EM radiation.

    Zz.
     
  8. Aug 17, 2015 #7

    blue_leaf77

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    I would say in most cases, the emission of photons always follow from some mechanism in which a change in energy of the atomic / particle system occurs - simply as a consequence of energy conservation. So, if atoms or molecules relaxate they will emit photons.
     
  9. Aug 17, 2015 #8
    What about when a molecule rotates? Or the stretching of molecular bonds? Would those occurances cause radiation? Two things examples of molecules radiating (and not the electrons and protons as the cause)...
     
  10. Aug 17, 2015 #9

    sophiecentaur

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    I thought a molecule was made up of electrons and protons? The actual energy of the photons depends upon the energy change involved.
     
  11. Aug 17, 2015 #10
    Yes, of course. I was just meaning the rotation and stretching of the molecule causing radiation. Not the charge from electrons or protons. Is radiation just caused from electrons and protons then? Not from rotating or stretching molecules?
     
  12. Aug 17, 2015 #11

    sophiecentaur

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    Bending and stretching and rotating a molecule alters the arrangement of the electrons and protons - what else? Changing the arrangement of charges will involve radiation. Why would you think otherwise? It is a pretty consistent model.

    I guess it's worth pointing out that a molecule that is rotating will only radiate (or accept) a photon if there is a permissible transition in its energy states. But that's the same as with an 'orbiting' electron.
     
  13. Aug 17, 2015 #12
    When in an atom, I was caught into thinking that the electrons jumping down energy levels was just the cause of radiation. But makes sense, then, that bending, stretching, and rotating of the molecules leads to the electrons emiting photons.

    So is blackbody radiation due more to the electrons continuously jumping down energy levels and therefore radiating photons, or from the aforementioned types of molecular movements causing the electrons to emit radiation? Or both?
     
  14. Aug 18, 2015 #13

    sophiecentaur

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    You seem to insist on talking in terms of 'electrons'. This will be because, as with everyone else, you got The Hydrogen Atom in your introduction to QM. Whilst everything contains electrons, the way it works is not as simple as The Hydrogen Atom. The energy states in a single atom all involved relatively big gaps and will not involve all frequencies of radiation. With molecules and densely packed matter (solids and liquids) there are many more combinations of charges and you have to include everything in a model to describe how the radiation interacts with the system. It's to do with the states of all the charged particles in a molecule, relative to each other that explains the frequencies that are absorbed and radiated. In condensed matter, you don't have discrete levels but whole continuous bands of energy, which is why you get continuous spectra and not lines in a 'red hot' object.
    Bottom line is that you need to not hang on to the simple models if you want to get to understand the more advance phenomena. For example, you cannot explain how light is reflected coherently from a shiny metal surface if you model it on one photon exciting one electron on the metal surface and then a photon being re-emitted. That would never produce a specular reflection because of the random delay in the absorption / emission process. It has to involve a large region of the surface, with gazillions of electrons involved.
     
  15. Aug 18, 2015 #14
    I appreciate your expanding on this matter. I feel as if I have a better understanding.

    So would it be fair to say, then, that, thermal radiation from an object (not from an ideal blackbody) has little or nothing to do with electrons jumping or changing energy levels in an atom? It would be all or mostly from rotation and vibration of molecules (which, of course, involves the altered arrangement of electrons and protons)?

    Question #2: Let's take a brick in the sunlight. If you were to say that it's the molecules of the brick that absorb the photons, couldn't it be said that, technically, the electrons are absorbing them as well since the molecules contain electrons?
     
  16. Aug 18, 2015 #15

    ZapperZ

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    Unfortunately, this is a very common theme here in this forum, and it requires another explanation why we have such a field of study as solid-state physics and condensed matter physics.

    When atoms and molecules are clumped together into a solid, they form a conglomerate in which the characteristics of the solid are predominately due to the collective behavior of all these atoms and molecules. What that means is that the individual behavior of the atoms and molecules are often no longer apparent in the properties of the solids.

    One important emergent, collective property that a solid has that are not found in isolated atoms and molecules is presence of phonons. This is a vibrational modes of a solid due to the gazillion atoms and molecules that make up the solid. In fact, phonons are responsible for a large range of properties of the solid, ranging from optical to thermal to electric transport.

    The reason I brought this up is because if you learn solid state physics, and especially the http://www.uni-tuebingen.de/meso/ssscript/phononen.pdf [Broken], you will discover that the phonon or vibrational modes have two branches - the acoustic and the optical modes.

    The optical modes are "electromagnetically active". It is basically a dipole mode, and this mode can not only absorb EM radiation, but can also, under the right condition, emits EM radiation (it is often called Raman active). This property is often attributed to why we see colors from different objects.

    So when you heat, say, a tungsten wire in an incandescent light bulb, if you look at the spectrum using the spectroscope that we give students in many intro physics laboratory, you will see a continuous spectrum of color, instead of a set of discrete lines that is expected from atomic transition. The heat causes all these vibrational modes of a solid to be enhanced to the point where the EM radiation they emit is intense enough to be seen. This is why the spectrum of light given off is different than from atomic gasses.

    Moral of the story here: More Is Different (Phil Anderson)!

    Zz.
     
    Last edited by a moderator: May 7, 2017
  17. Aug 18, 2015 #16

    sophiecentaur

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    You can say what you like. Your version has some aspects of truth in it. But if you insist on your particular, partial explanation then you will find it harder and harder to explain all the EM absorption and emission phenomena there are.
    Instead of trying to hang on to your basic idea, seemingly at all costs, I recommend that you make an effort to learn a bit more Physics. Why do you come to PF if you just want to be 'right' all the time?
     
  18. Aug 18, 2015 #17

    I more than appreciate your response. I've heard, though, that a real-life blackbody (not an ideal or perfect one) will show spectral lines. And I've heard that some studies in astronomy use this kind of blackbody analysis--e.g. relative intensities and spectral lines--to infer information about stars. Perhaps I'm missing something big here. But, if I'm understanding you correctly, thermal radiation, from objects, would just involve molecular vibrations and not jumping of electrons? Or can it be both for when it comes to a non-ideal blackbody?
     
  19. Aug 18, 2015 #18
    In no way am I trying to be right. I am just trying to get a full understanding and throwing some things out there for analysis. I really appreciate the long answer you gave me and I feel it has helped. Was just wondering about that one other way of looking at it, that's all.
     
  20. Aug 18, 2015 #19

    sophiecentaur

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    I think you may be referring to the absorption lines, caused when the black body light emitted from a star shines through its outer atmosphere (rarified gas with single atoms or molecules where the energy levels are discrete). The Helium Lines were spotted in sunlight - for the first time and that was why the name Helium was chosen for the newly found element, or so I believe.
    New phenomena need new ways of looking at things and the simple 'electrons radiate EM' approach is just not enough. Do a bit more reading round instead of endless Q and A, which can be very inefficient use of your time if you want the bigger picture.
     
  21. Aug 18, 2015 #20
    Yes, thank you for clearing that up about absorption lines. And, I hear you about reading up more. Let me just close by presenting you one other thing.

    What may have been confusing me in regard to whether it's the electron jumps or molecular rotations and vibrations causing the emission of photons is the following (received from an associate):



    'In solids, liquids, and gases simple scattering of the molecules transfers the kinetic energy of one molecule to the potential energy of another, i.e. raises an electron to a higher level. The electron goes back to its ground state releasing a specific photon, or a cascade of photons, depending on the energy. Remember that the higher levels with respect to n, the radial quantum number, are closely packed. These photons are the ones emitted as blackbody radiation, and they are a continuum because of the 10^23 molecules per mole and the almost continuous energy levels.'

    The part that says, 'The electron goes back to its ground state releasing a specific photon, or a cascade of photons', seems to contradict the idea of blackbody radiation and a continuum, and of molecular vibrations as being the cause for emiting.

    If you can shed any light on that quote, I'd be more than grateful.
     
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