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Continuous emission spectrum

  1. Nov 16, 2013 #1
    why does the continuous emission spectrum depends only on the temperature of the solution and not on the characteristics of the source?i could not understand this.someone please explain me this:uhh:
     
  2. jcsd
  3. Nov 16, 2013 #2

    Simon Bridge

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    Welcome to PF;
    What is it about this property that you don't understand?

    It is usually gaps in the spectrum that depend on the material properties.
     
  4. Nov 16, 2013 #3
    It is usually gaps in the spectrum that depend on the material properties.

    how can u say that it usually gaps?it consists of unbroken luminous bands of all wavelengths of colours from violet to red.:confused:

    also please explain why the spectrum is independent of the characteristics of the source.
     
  5. Nov 16, 2013 #4
    there are no gaps in the spectrum that is why its called continuous
    :confused:
     
  6. Nov 16, 2013 #5

    ZapperZ

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    It would be nice if you cite a specific example of the type of source.

    In an incandescent light bulb, for example, the light produced is due to the vibration of the lattice ions, which corresponds pretty much to the phonon density of states, which is "continuous". When you have a large bulk material, such as a wire, the individual characteristics of the source tend to get washed out (there are exceptions, such as the varying intensity of the spectrum at different wavelengths, for example). So at such high temperatures, only the thermal vibration matters most.

    Zz.
     
  7. Nov 16, 2013 #6

    Simon Bridge

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    The Sun shows a continuous spectrum with dark lines in it.
    Technically it's not continuous but you were asking about how the material properties affect the spectra.


    Because it is a very complicated material and/or it has a high temperature compared with the condition required to see a material-dependent spectra.

    Consider the hydrogen and helium atomic spectra - compare with the solar spectra (the Sun is almost all hydrogen and helium).
     
  8. Nov 16, 2013 #7
    carbon arc could be an example
     
  9. Nov 16, 2013 #8

    solar spectrum is an absorption spectrum.i am talking about emission spectrum
     
  10. Nov 17, 2013 #9

    ehild

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    A single atom has discrete energy levels and its emission spectrum consist of discrete lines of frequency ΔE/h.
    In a two-atomic molecule, the atoms interact, they make a system. Pauli's principle forbids two electrons be in the same state. The energy levels split, you have twice as many levels as in case of a single atom. In a solid or liquid, there are many interacting particles, and the energy levels split to many new ones. You have so many transitions, and emitted/absorbed photons with similar frequencies, that the spectrum looks continuous. But it reflects the material behaviour. It is only the case of "black-body radiation" where the emission at a given wavelength or frequency is function of the temperature alone. But a black body is a special hypothetical material (or set-up) which absorbs all incident radiation. Real materials do not do that, but the emission spectrum of some ones can be approximated by the black-body spectrum or proportional to it.

    ehild
     
  11. Nov 17, 2013 #10

    Simon Bridge

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    Anything that emits light has an emission spectrum.
    Here is the emission spectrum for the Sun:

    320px-Spectrum_of_blue_sky.svg.png

    ...it is close to that of a blackbody with a temperature of around 5800K.

    The dark lines in the solar spectrum are the absorbtion spectrum of the individual elements that the Sun is made of. You see them as sharp troughs in the curve (pic. above).

    The carbon-arc spectra also shows a fine structure.

    jphysd467924f02_online.jpg

    (Note: The width of the spikes owes more to the detector than the source.)

    http://pdf.directindustry.com/pdf/a...-weather-ometer-brochure/27780-169323-_3.html
    ... a brochure comparing spectra for different types of lamp with the Sun - including a carbon-arc lamp - for a much wider range of wavelengths. On this scale the fine structure is not apparent.

    Also see: http://www.ispc-conference.org/ispcproc/ispc19/696.pdf]Optical[/PLAIN] [Broken] emission and absorption spectroscopy of carbon arc plasma in the formation of carbon nanotubes.


    There must be better examples somewhere...

    So, anyway, what is the property of the carbon-arc spectra you need explained?
    What sort of spectra would you have expected and why?
     
    Last edited by a moderator: May 6, 2017
  12. Nov 17, 2013 #11

    CWatters

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    What ehild said.

    Didn't this come up in a similar thread only the other week?
     
  13. Nov 17, 2013 #12

    Simon Bridge

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    I think there are multiple misunderstandings here - like what a spectrum actually is, what it represents, that you can have absorption and emission spectra at the same time, the contributing factors to the intensity vs wavelength curves, how spectral lines form, stuff like that.

    It would be useful to know what level this question is being asked at.

    ehild basically shows how you get energy bands in solids: they are extremely close energy levels - transitions between very close levels will look continuous unless you try quite hard to look carefully ... and you get HUP limits too, so the "discrete" energy levels are actually a range of possible values. Get the energy levels close enough together and they overlap.

    You can get transitions between two bands though ... the range of available energies is what gives an LED spectra it's broad shape (I mean without the lens or any coatings that manufacturers add later).

    There's another effect - where you have transitions from the continuum: i.e. unbound states.

    (Simplifying somewhat...)
    When an electron is no longer bound to an atom, it may still be contained within the apparatus.
    We can model the apparatus as an infinite square well (say) so there are still discrete energy levels to transition from... it's just that they are much closer together than the atomic energy levels. i.e. The atom has dimensions of order 1A, while the apparatus likely has dimensions of order of 1cm = 100000000A so we can expect the energy levels to be 100-million times closer together. The prev-mentioned HUP limit works here too.

    In a plasma - for instance - the media is so hot that the atoms are totally ionized. The only available transitions will be between these continuum states rather than atomic states ... so the only effect on the spectra will be the distribution of particles between these states ... which is pretty much the definition of "temperature".

    Therefore we'd expect the spectra of a plasma to be continuous and to depend only on it's temperature.

    It's a pretty big subject.
    Which is why it is useful to narrow it down.

    I suspect OP is just asking how come you get a full rainbow when you put light from a carbon arc through a prism, but not when you put light from hydrogen or mercury ... the answer will be that the prism presumably used was just not an accurate enough instrument to separate out the spikes in the carbon-arc spectra ... presumably it wouldn't be able to separate the orange peaks in the sodium spectra either.
     
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