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Hot object and IR emission

  1. Jan 27, 2010 #1
    Why does a hot object emit Infra red light. How are photons are emitted from it?
  2. jcsd
  3. Jan 27, 2010 #2
    light is emitted when the atom jumps from excited state to ground state,when the object is heated,heat energy is supplied to the object,which causes rearrangement of the electrons,this occurs when the atoms are in excited state ,they come back to the lower energy level
  4. Jan 27, 2010 #3
    This is probably not the correct mechanism for infrared emission.

    For one thing, this explanation seems to require that the object is immersed in an existing bath of radiation; in fact, a black body emits just as much energy if it is located in the deepest depths of interstellar space, surrounded by the cold of absolute zero.
  5. Jan 27, 2010 #4
    you may be right ,i was just thinking of the light but not IR emission.
  6. Jan 27, 2010 #5


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    IR is usually associated with vibrational energy. There are vibrational modes of various energies and the transition between them either requires the absorption (heating) or requires the emission (cooling) of a suitable frequency of the EM radiation. Electronic transitions are usually associated with visible and UV scale energies and superimposed upon those excited states are the vibrational IR frequencies as well.

    Just how energy is converted into photons escapes me. If you ever figure it out, let us know!
  7. Jan 27, 2010 #6
    In a solid at rest, there is positive charges cancel the negative charges. In a mechanically vibrating solid, it seems unlikely that the total positive charge centers (the nucleii) and the total negative charge (the electrons in orbitals and whatever free electrons are present in a metal) would necessarily be vibrating exactly in sync. If there is any net lag between the vibration of the positive and negative charges, then there would be a net vibrating charge which ought to radiate.
  8. Jan 28, 2010 #7
    Net vibrating charge which ought to radiate.
    At what frequency should the charges vibrate so they radiate IR? Should it be in terahertz? Can heat make the charges vibrate at such high frequency.
  9. Jan 28, 2010 #8
    This should be a good check on the theory. Let's take a solid with atoms 1 angstrom apart and a speed of sound equal to 1000 m/second. A wavelength of 1 meter would then have a frequency of 1000 hz, so a cube of 1/2 meter would support a lowest-mode standing wave vibration of 1000/hz. (I'm not going to be really careful about compression waves versus transverse waves.) The very tiny cube of eight atoms is effectively smaller by a factor of 5x10^9, so it supports a vibration frequency of 5 x 10^12. We can use hw = kT to arrive at an equivalent temperature of what comes out for me to around 200 degrees Kelvin, fairly close to room temperature. What does this mean??

    It's a bit of a coincidence that the numbers come out this way, because room temperature shouldn't really figure directly in the problem. However, there is something called the Law of Dulong and Petit which deals with the specific heats of solid. It tells you that the molar thermal energy of a typical solid is theoretically 3RT, which is justified on account of six modes of energy (3 vibrational and 3 kinetic) for each lattice atom. The law breaks down at very low temperatures for a reason which Einstein first pointed out: the eventually, the very highest frequency modes are "frozen out" by quantum mechanics. The point at which this freezing out process starts to kick in is roughly given by the relation I used in the previous paragraph, hw=kT.

    So I've described a solid for which the Law of Dulong and Petit starts to fail around room temperature. Is this realistic for an actual solid? For most metals, the specific heat is good down to cryogenic temperatures. But I've chosen a sonic velocity which is perhaps a little low. Let's go to Wikipedia and get some numbers....

    OK, after checking Wikipedia it seems I'm not that far off. They basically do what I've done (well, they include some correction factors that I've glossed over) in the article on "Debye Model" and there what I've calculated is called the Debye Temperature, and it's surprisingly high; definitely in the range of room temperature for ordinary metals. (The specific heat doesn't depart radically from the expected law until you get down to about 1/3 of the Debye Temperature.) I actually was six times too low on the sonic velocity but also three times too low on the lattice spacing; these errors tend to cancel out giving me a ballpark factor of 2 error. So I'm thinking I've got it reasonably correct.
  10. Jan 28, 2010 #9

    Claude Bile

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    Conway is on the mark;

    Objects whose temperature is above 0 K possess thermal energy. Thermal energy, on a molecular level is simply the kinetic + potential energy of the particles. That is, in hot objects, the atoms that make up the object tend to wiggle around.

    Now, in the process of wiggling around, negative charges displace from positive charge forming dipoles. Oscillating dipoles emit EM waves, which is why objects emit black-body radiation.

  11. Jan 29, 2010 #10
    i understood what you said, can you please clarify me how 'Oscillating dipoles emit EM waves'
  12. Jan 29, 2010 #11
    For purposes of discussion, this is the kind of thing that most people probably just assume as a convenient starting point. The actual mechanism isn't that easy to explain but it's basically the same principle that makes a radio transmitter work.
  13. Jan 29, 2010 #12
    NO!,not for that purpose(discussion),i really just wanted to know
  14. Jan 29, 2010 #13
    Sorry, but I'm not sure anyone is going to be able to explain this. I'm not about to try.
  15. Jan 29, 2010 #14
    i was trying to search on the web but didn't find it
  16. Jan 29, 2010 #15


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    Hi physixlover! :smile:

    The electrons move, and that causes electromagnetic waves …
    See http://en.wikipedia.org/wiki/Dipole_radiation#Dipole_radiation" for the maths.

    Where do the photons come from?

    Well, the moving charges produce a field, and this is one of those cases where it's really the wave rather than the particles that are created, and the particles somehow condense out of the waves. :wink:
    Last edited by a moderator: Apr 24, 2017
  17. Jan 29, 2010 #16

    Claude Bile

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    From a classical viewpoint;

    A dipole is a pair of opposite charges (+ and -). When these charges move in a, say sinusoidal motion, the electromagnetic field surrounding these charges also varies sinusoidally with time.

    The wave equation then dictates that these sinusoidal variations must propagate outward in space, these are EM waves.

  18. Jan 29, 2010 #17
    thanks for clarifying my doubt
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