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B Why do objects glow

  1. Apr 7, 2017 #1
    well, this may be obvious for many of you guys here! and I know it must be! but I don't seem to find any satisfying answer online.

    well, I know that when you heat up any kind of matter it will emit light depending on its
    temperature, for example, humans will emit infrared light the same as metal, but when heated enough it will glow so it will emit visible light, I never actually asked myself why this happens, not until now!

    because at school we studied energy levels in an atom, and talked about how when you shoot electrons at these atoms their electrons will "get excited" and jump to another energy level, but soon after they will get back and in this process they will emit radiation and we will get what's called an emission spectrum,
    so the thing is, this emission spectrum is specific to atoms and they only emit specific radiation,
    so how come all matter radiate the same way (infrared and when heated visible) but are made of different atoms??
    (this may seem very basic to most of you people here, and that's why i hesitated before posting it, but it really bothers me not knowing what's really happening or not finding a satisfying answer online)
  2. jcsd
  3. Apr 7, 2017 #2


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    All solid or liquid objects have pretty much similar set of microscopic vibration modes, which produces a continuum thermal emission spectrum, as compared to the line spectrum of individual atoms or molecules in a gas.
  4. Apr 7, 2017 #3


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    The discrete energy levels of an atom are for an isolated atom. When it is part of a molecule, you have to consider the molecule as a whole. In a solid, the discrete levels of atoms get combined into bands, where a continuum of energy is possible.

    In gases (where the individual atom approach is not bad), you also have to consider other things such as the Doppler effect and collisions, which will broaden the discrete energy levels.
  5. Apr 7, 2017 #4


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    Actually, all things will not radiate the same way, for the reasons you indicated.

    Black bodies are idealized objects that have a smooth emission spectrum which only depends on the temperature. But most objects are not perfect black bodies. The Sun is somewhat close, but there are various gaps due to absorption lines in the hydrogen spectrum and the spectra of other elements. We aren't very close to black bodies (in the physics sense) at all.

    That said, the other responses are good. In the infrared part of the spectrum, there are a lot of molecular transitions available. The more transitions there are, the fewer gaps in the spectrum, and it starts to approach the black body spectrum, at least in the infrared part. But you still have big deviations from black body for some materials, such as reflective metals.
  6. Apr 7, 2017 #5
    thanks for the quick reply!
    correct me if I'm wrong!
    a dense object (liquid or solid per say) do not have an emission spectrum made of individual specific radiations (seen as lines), they have continuum thermal emission spectrum which does not have as many gaps and therefore can radiate in so many different parts of the electromagnetic spectrum (starting in the infrared part and going up to visible) .
  7. Apr 7, 2017 #6


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    Yes, if you take an IR spectrum of a gaseous sample, you will see spectral lines that are somewhat broadened by Doppler effect and contain a rotational structure (which means that the lines actually consist of several closely spaced peaks). If the sample is a liquid or a solid, the interactions between the molecules increase the number of possible microscopic energy states so that the spectrum becomes continuous, like in the image below (but can still contain peaks).

  8. Apr 7, 2017 #7
    big thanks to you guys! :D this was too helpful.
    thanks hilbert2 !!
    you too DrClaude and Khashishi !!!
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