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Why do different frequency EM Waves behave different ?

  1. Jun 14, 2008 #1

    JPC

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    Hi, i was wondering
    Sorry if i dont have the right words, i study in France

    Why are Microwaves better for heating up food ?
    I mean since E=hv, these should contain less energy
    Right now, i know vaguely, that atoms absorb photons where the energy can make it go from one of its characteristic Energy states to another.
    Is it that lower energy photons are compatible with an energy state leap of more atoms ?

    And how can some Lasers actually decrease the temperature (energy) of a material ?
    I have only seen Leaps from E0 (fundamental status) to E=0
    How does this work ?

    And with radios used in medecine to see bones.
    Why do higher frequency waves 'go through' more materials ?
    Is it because they do not correspond to many energy status leaps of different atoms ?
    If this is so, we would have used very low waves too, so doesnt make sense

    And, how does refraction and reflection work in quantum mechanics ?

    IF this is all too long for you to answer, is there a website which can explain we all about waves in quantum physics (without going into very hard details, i have just finished high school)
     
  2. jcsd
  3. Jun 14, 2008 #2

    malawi_glenn

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    Water have high absorption in the Microwave part of the spectrum, compare with absorption lines of atoms, only certain wavelenghts of light can be absorbed.
    http://en.wikipedia.org/wiki/Absorption_spectrum
    http://www.lsbu.ac.uk/water/vibrat.html
    http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/A/ActionSpectrum.html


    And about radio waves in medecin, it is not the radio waves that makes the image, but the radio frequence correspond to the frequency of the nucleis spin, so that the spins of the nuclei will align with the radio-field. Then one uses a big magnet to localise where these nuclei are in the body. The nuclei one wants to see are the hydrogen in water, and bone does not contain water. Read more about this here: http://en.wikipedia.org/wiki/Nuclear_magnetic_resonance
     
  4. Jun 14, 2008 #3

    Dale

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    There a lot of different medical applications of EM energy at different frequencies. It basically depends on what you want energy delivered to or from (or what you want to avoid).

    ELF-SLF, good for exciting nervous and cardiac tissue, used in pacemakers, defibrillators, EEG, ECG, and DBS

    VLF-MF, good for not exciting nervous tissue even with large currents, used in RF ablation and electrosurgery

    HF-UHF, not absorbed much by body tissues, corresponds to proton Larmor frequency at reasonable field strengths, used in MRI and communication

    Infrared, widely absorbed emitted and scattered by tissues, used in thermometers and pulse oximeters

    Visible, widely scattered by tissues, used in OCT, external (visual) diagnosis, and optical activation targeted drug delivery

    Ultraviolet, absorbed by tissues and can be ionizing, used for sterilization, neonatal jaundice?, vitamin D therapy

    X-rays, absorbed by bone, used for imaging (x-ray, CT)

    Gamma-rays, not absorbed by much, used for imaging (nuclear medicine)
     
  5. Jun 14, 2008 #4

    JPC

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    oh, so a Microwave is just sending waves that are absorbed by water
    hopefully its everywhere in our food.
    Puting dry materials in microwave may have limited effects then ?
    and why metals react violently to microwaves ?

    Also, i was wondering, how come some objects are dark or black ?
    - i mean, if the photons are not absorbed they should continue their way through the material, like water (transparent)
    - and when they are aborbed , the atoms must go from a energy state to a higher one, and then re-emit photons to come back to their Fundamental energy state.
    So it seems that black/dark materials absord photons, but only release the energy through heat ?
     
  6. Jun 14, 2008 #5

    malawi_glenn

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    http://en.wikipedia.org/wiki/Microwave_oven

    the colour of a material is the one that the material reflect/does not absorb. A black surface absorbs everything, and a white nothing, and a red everything except red.
     
  7. Jun 14, 2008 #6

    JPC

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    yes, but once a black material has absorbed the photons, he must re-emmit photons to comeback to its fundamental energy level ?
    are this emmited in non-visible ?

    What I was trying to understand is the quantic difference between transparent and opaque objects .
    and how does the atoms reflect the photons they don't absorb? I thought they would just pass through
     
  8. Jun 14, 2008 #7

    Dale

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    Usually it is re-emitted in the infrared
     
  9. Jun 15, 2008 #8

    malawi_glenn

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    exactly, solid bodies behave different than single atoms (gas), solid bodies can in many cases be approximated by the "Black Body"-concept in thermodynamics.

    JPC: An object is transparent if it for a certain wavelength "kills" the beam of photons, i.e absorb it, and that the 'optical depth' is much smaller than the size of the body.

    for example, my bible here on my desk is opaque to visible light, since I cant see right through it. But if I would have access to a X-ray source, the X-rays would go right through it.

    http://en.wikipedia.org/wiki/Optical_depth
     
  10. Jun 15, 2008 #9

    JPC

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    I dont understand
    i thought transparent objects dont absorb anything

    Like for example, if you have a white solid and, transparent solid, a black solid.
    and if we consider a beam of white light

    - In the black one, the photons are absorbed and the atoms move to a higher state of energy, and then release that energy by emmiting non-visible frequency photons
    - In the White one , none are absorbed, but how come the photons change direction, and are emited in every direction ? is it that actually the atoms absorbs them, but re-emmits them straight away with the same frequency ?
    - And now what happens in a Transparent object exactly to photons ?
     
  11. Jun 15, 2008 #10

    malawi_glenn

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    Transparency is relative a certain wavelenght, the EM-spectrum is more than just the colours we see with our eyes.
     
  12. Jun 16, 2008 #11

    JPC

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    i dont understand what you mean
    any webpages that explain this in details ?

    i know that the visible bandwidth is very narrow, but "Transparency is relative a certain wavelenght"
     
  13. Jun 17, 2008 #12

    Dale

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    Google "absorption spectrum" especially the absorption spectrum of water.
     
  14. Jun 17, 2008 #13

    malawi_glenn

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    Absorption is a function of the wavelenght, i.e the transparency is afunction of the waveleght. And this function differs between the different materials.
     
  15. Jun 19, 2008 #14

    JPC

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    but what inside a atom or material characterizes if it will be transparent or opaque ?

    i mean for absorption, we know that it is the different characteristic energy levels of the atom with E=h*v
    but for transparency ?
     
  16. Jun 20, 2008 #15

    ZapperZ

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    Hum... I'm beginning to feel like a parrot, but you might want to read our FAQ thread in the General Physics forum.

    Zz.
     
  17. Jun 26, 2008 #16

    JPC

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    But its not easy to find the specific Thread
    the Website should offer a 'search posts containaing the text you want' utility

    But i was wondering about one application for the millitary
    for spies : a material opaque to visible light, but transparent to IR
    and they could wear IR to spy safely behind a wall made of this material

    are there any more realistic applications ?
     
  18. Jun 26, 2008 #17

    ZapperZ

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    The FAQ is one of the Sticky threads at the TOP of the General Physics forum. What is there to find?

    Zz.
     
  19. Jun 26, 2008 #18
    First let me admit that I'm not a quantum physicist but I AM an electrical engineer and also a bit of a physics junkie. However I don't see the clear answer in any of the replies to your posts. So here is my understanding of it and I believe that I am right.

    Any molecule will absorb a photon when the wavelength of the photon matches the size of the molecule. In radio communications, an antenna must be cut to the wavelength of the frequency you want to receive. So it works somewhat the same way with photon absorption in materials. Now when, say, a black piece of cloth absorbs photons, it's aborbing all the photons in the VISIBLE spectrum. These wavelenghts are MUCH longer than the size of an atom. So we understand that the molecules, which are much larger, are aborbing the energy resulting in an increase in heat energy, not electron excitation.

    Longer wavelengths such as AM radio and VHF/UHF television are so much longer that none of them are absorbed by molecules in the body.

    When the photon wavelength is approximately the size of the atom, or perhaps when the photon energy corresponds to a quantum level of the atom, the photon will be absorbed and elevate an atom's electrons.

    Microwaves in an oven work because the wavelength corresponds to the size of the water molecule.

    Again, I'm not a quantum physicist but I'm very confident that I have the general idea.
     
  20. Jun 27, 2008 #19

    ZapperZ

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    But again, if you read the FAQ, you'll see what's wrong with this. For example, why would the size of an atom matters in the case of, say glass versus quartz. I still have essentially silicon atoms making up both material. Yet, in one case, glass absorbs UV light, while quartz allows almost all of the UV light to pass through. So already this tells you that it really isn't about the nature of the atom or the size of the atom, but rather how these atoms are arranged. In the FAQ, the phonon structure plays a major role in such a process. How the atoms are arranged dictates such phonon structure. This is the collective behavior of the material in which individual atoms properties are no longer dominant.

    Zz.
     
    Last edited: Jun 27, 2008
  21. Jun 27, 2008 #20
    Yes but I said a MOLECULE, not just the atom. I've never heard of phonons in a physics context before (phonon is also a term for the most basic sounds of speech) but what you're saying is that the molecular structure differences account for the absorption differences. I think that's basically what I said. It's going to depend on the size of things because that's the way em radiation works. I suppose the lattice structure of the glass must resonate in 3 dimensions with the UV.

    I'll look around and try to find those FAQ's. I'm new to this forum and I don't even know what a "sticky" thread would be. But I do appreciate your time and your obvious extensive knowledge of the subject. Thank you.
     
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