Is it more than IR that contributes to "warming"?

Discussion in 'Physics & Math' started by Oystein, Dec 9, 2016.

  1. Oystein Registered Senior Member

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    When I stand out in the sun and feel its "warmth", I know that most of that warmth come from the infrared frequencies of the EM spectrum. But, do other frequencies of the ER spectrum contribute to "warmth"? Starting from the longest wavelengths: I'm sure that radio waves don't contribute. But how about microwave frequencies? Do some microwave frequencies get thru the atmosphere and contribute to the heat I am feeling? How about visible light - does it contribute to the heat I am feeling? Getting into the smallest wavelengths: do UV frequencies contribute some to the heat since I know some UV frequencies get thru (sunburn)? I'm pretty sure that x-rays and gamma rays don't contribute much because if they did I would probably be dead very shortly.
     
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  3. Oystein Registered Senior Member

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    I guess that whenever my body absorbs a photon, be it of any frequency of EM wave, then the energy in my body has increased (a very, very tiny bit

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    ), even with a radio wave photon.
     
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  5. billvon Valued Senior Member

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    Of course.

    Photons that hit your body can do several things. At a basic level they can be reflected or absorbed, and this goes for any wavelength - although some are more likely to be absorbed than others depending on things like skin color. The photons that are absorbed heat the tissue they are absorbed by. From there some is re-radiated as IR radiation, cooling the body (although never as much as the original photon warmed it.)
     
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  7. Oystein Registered Senior Member

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    I guess our skin's ability to sense infrared radiation is an evolutionary thing -- keeps us (and other mobile living things) from stepping in the lava.
     
  8. exchemist Valued Senior Member

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    Good points already made by others. But I think your question is interesting, because it is not immediately obvious how a UV or visible-frequency photon, which would be absorbed by exciting an electron in an atom or molecule, results eventually in a heating effect.

    With IR or microwaves it is obvious, because these directly stimulate vibrations and rotations of molecules, i.e they lead to increases in the thermal motion of the molecules, which we call heat. With electronic absorptions, molecules are put into an excited electronic state. Normally, one thinks of such an exciting state dropping back to the ground state by re-emitting a photon. Neither process would immediately be thought to increase thermal motion. But in fact it can. One reason is that the equilibrium bond length in the excited state of a molecule will generally not be the same as that in the ground state, because a bonding electron may have moved to a different orbital. Say the bond becomes weaker, i.e. longer, in the excited state. Since atoms do not move significantly during the absorption process (Franck-Condon Principle), the atoms will be closer together after excitation than their new equilibrium bond length, and, "boing", they will spring apart - and continue to vibrate. In other words the electronic excitation has put the molecule into a vibrationally excited state as well. This adds thermal energy. This same electronically excited molecule may then lose energy via collisions and later may emit a photon from its vibrational ground state. If it does, the atoms will be further apart than they should be in the ground state, and again, "boing" they will be pulled together, and continue to vibrate. Again, energy has been converted to vibration. This is one of the processes (there are others) by which electronic excitation energy is progressively converted to heat.

    I am not totally sure what the absorption of radio waves involves at the molecular level, but I would guess it would be translational motion, i.e. making the whole molecule move. This again is one of the modes of thermal motion and would directly contribute to heating.
     
  9. Oystein Registered Senior Member

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    So, can I summarize by saying that all EM radiation contributes to an energy increase but IR is the largest contributor due to the chemical (atomic) makeup of the surface of our bodies?
     
  10. Oystein Registered Senior Member

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    Any comments on this observation?
     
  11. exchemist Valued Senior Member

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    Not quite. I would rather say that:

    (a) almost all* the solar radiation hitting our bodies is turned to heat, simply because that is what would happen in the case of most materials made of molecules, and

    (b) IR is the largest contributor because the greatest proportion of solar radiation at the Earth's surface is in the IR region of the spectrum. See this distribution curve : https://en.wikipedia.org/wiki/Sunlight#/media/File:Solar_spectrum_en.svg
    (Red areas of the graph are sunlight at the surface, yellow is sunlight at the top of the atmosphere, i.e before any absorption or scattering occurs.)


    * I say "almost all" because a tiny fraction of the light takes part in photochemistry in our skin instead, e.g. the chemical reactions that synthesise vitamin D and those that cause skin to tan or burn.
     
  12. Oystein Registered Senior Member

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    Danke.
     

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