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Relative Abundance of Light Frequencies?

  1. Dec 16, 2012 #1
    I was curious if anyone had ever seen information about how often one frequency of electromagnetic radiation appears in the universe compared to the other. What is the most common frequency or range of frequencies, etc? Is there a way to even estimate this?
     
  2. jcsd
  3. Dec 17, 2012 #2
    microwave dominates everything as that's the cosmic background.
     
  4. Dec 17, 2012 #3
    Ah, of course. How about second most abundant?
     
  5. Dec 17, 2012 #4

    mfb

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    In terms of photon number, I would expect it to be ordered by frequency: More microwave photons than infrared, more infrared than visible, more visible than UV and so on.

    In terms of energy, high-energetic photons have some advantage, of course. Visible light <-> infrared might be interesting. UV, X-rays and gamma rays are rare, I would not expect that the order changes here.
     
  6. Dec 17, 2012 #5

    sophiecentaur

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    An object (so called black body) at any temperature will emit a spectrum of em radiation. This link shows how the spectrum varies with the temperature of the emitter and there are many more you can find. There is a maximum wavelength for each spectrum.
    There are a very large number of bodies, all at different temperatures (stars, gas clouds, rocks etc.) and they all will be producing different spectra with different maxima (plus they will all be constantly absorbing radiation from elsewhere). There will be an effective 'representative' /mean temperature if you look in all directions from here which is actually pretty cold (about 3K) which is the Cosmic Microwave Background Radiation which is arriving from all directions and that implies a wavelength of around 1.8mm (long infra red).
    On Earth, we 'see' a mean temperature of about 300K (dominated by the nearby Sun, of course) but most places in the Universe are nowhere near a hot source so the mean temperature, seen from an 'average location' in the Universe will probably be not far above the CMBR temperature . I think this statement must be justified on the grounds that, even from Earth (well within the Galaxy) the CMBR has been measured with some confidence - so, if even from our position, we can 'see' a significant amount of 'really empty space' then that's what you would see, all around you, at most locations in the Universe.
    Is there another factor that I have left out - something obvious, to do with the statistics, perhaps?
     
  7. Dec 18, 2012 #6

    mfb

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    Why? The Planck spectrum does not have one, and I don't see any reason to expect a maximum wavelength.
    At the other side - short wavelength - there is a sharp drop at a temperature-dependent value (if the spectrum is expressed as function of wavelength), but that is not a minimal wavelength either.
     
  8. Dec 18, 2012 #7

    jtbell

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    I suspect sophiecentaur meant to say something like "maximum-intensity wavelength" i.e. the wavelength of the peak of the blackbody distribution.
     
  9. Dec 18, 2012 #8

    sophiecentaur

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    Right. I was gibbering a bit. I meant the spectral peak (broad, of course).
     
  10. Dec 18, 2012 #9
    No, during the day the sunlight we see has a temperature of ~5850K or so (from the temperature of the Sun's photosphere). The Earth's surface is roughly 300K in some regions, so the Earth glows infrared, but the power from the Sun (during the day) greatly exceeds the Earth's thermal glow.
     
  11. Dec 18, 2012 #10

    sophiecentaur

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    I was merely arguing that we are (obviously) in thermal equilibrium. We must be losing the same average power that we are absorbing and that means an average effective temperature of around 300K (A simple model involving the Earth being a black conducting ball with no atmosphere, of course). The vast majority of the energy we receive is from the Sun so it 'dominates' our resulting temperature. In most other locations in space, our temperature would be only a few K.
     
  12. Dec 18, 2012 #11

    mfb

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    We see ~6000K in a very narrow solid angle. If you average the flux over the full hemisphere, you get a value which is a bit above 300K.

    (radius of sun)/(distance to sun) is ~0.005, so the sun covers 0.005^2 of the sky. Radiation scales with T^4, so we would receive the same radiation (~1.3 kW/m^2, but with a completely different spectrum) if the whole sky would glow with 410K. Not the whole surface is perpendicular to the solar radiation, this gives a lower equilibrium temperature. In addition, earth is not a perfect black body, of course.
     
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