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Trying to understand what happens when EMR goes through matter (or doesn't)

  1. Aug 21, 2011 #1
    Sorry for the length of this post, but even if you can only help answer one of my questions I'd be grateful! After a lot of reading about electro-magnetic radiation's interaction with matter, I am still unclear about a few things. Let me first explain what I've gathered from my reading so far, just in case I've misunderstood something basic. How EMR interacts with matter depends on many things. One thing it depends on is the energy of the EMR, with long radio waves having the least energy and gamma rays having the most. In order of ascending energy, this is how EMR can interact with matter:

    A: Lowest energies (~radio, microwave): EMR can change the rotation or torsion states of matter. (Is this with radio waves AND microwaves? or just microwaves?) These are discrete, quantified states, not a continuous scale of less energy to more energy.

    B. Low energies (approx. ~infrared): EMR can change the vibration states in the bonds. These are also discrete and quantified.

    C. High energies (approx. ~visible light, UVA): EMR can excite electrons in matter to higher states (electron shells). These electron energy levels are also discrete and quantified.

    D. Highest energies (approx. ~UV, X, Gamma): When the energy level exceeds the highest available electron state, EMR can directly eject an electron from an atom (ionization).

    Question 1: A and B can both make matter warmer; consider a microwave oven, and an infrared lamp. Am I correct in saying that rotation, torsion and vibration are all considered forms of molecular kinetic energy and thus affect the perceived temperature? What about in C? Does excitation cause warming? How?

    Now suppose I take the example of a human.

    When a human is exposed to radio waves, apparently the EMR is transmitted right through the body and has no effect no matter what the intensity. Proof: our bodies don't block FM radio stations' signals and we don't get burned even when standing near a broadcast tower.

    Question 2: It seems easier to understand radio EMR passing through objects when you think of it as a wave. But if you think of it as particles, I don't understand how it can pass through matter unhindered. What if a radio photon happens to be directed right at the nucleus of an atom? Isn't there some sort of "collision"? What happens then?

    Question 3: I assume a large collection of humans would be able to block radio waves, just as a large volume of earth or concrete can block radio waves like in a long tunnel. My question is… where is this absorbed energy going? Apparently, radio waves are too low energy to change the rotational or torsional states of the atoms in the earth or concrete. There simply isn't enough energy to jump up to the next quantum state. So, what's happening to the radio wave energy when it's absorbed?

    When a human is exposed to microwaves, the EMR can be absorbed and converted to heat, or can be mostly transmitted. Proof: meat can be cooked in a microwave oven, but our bodies also don't absorb WiFi signals at the same frequency.

    Question 4: Is this just due to an intensity difference? If a microwave oven was set to a low enough intensity, it wouldn't cook our flesh? And if a wireless router's signal was boosted high enough, it could cook us?

    When a human is exposed to infrared, EMR is mostly absorbed by the body and converted to thermal energy; it is not transmitted. Proof: infrared sauna warms the skin; holding your hand in front of a remote control stops the IR signal's transmission. When a human is exposed to visible light, the EMR is mostly absorbed, not transmitted, and some is reflected. Proof: you can't see through a person, you only see what is reflected by their skin/hair. When a human is exposed to UVB, EMR causes ionization of atoms in the skin. Proof: sunburn. X-ray exposure can also cause tissue damage. Proof: radiation-induced cancers.

    Question 5: Unless the incoming X-ray or gamma ray exactly matches the ionization energy for an atom, there will be some excess energy left over. What happens to that? Is it transferred to the free electron as kinetic energy? Is the photon downgraded to a longer wavelength? What determines this?

    Question 6: When incoming EMR's energy matches an available quantum state in matter, it can be absorbed. However, if it doesn't match, there are still two possibilities: transmission, or reflection. What determines what will happen to this EMR?? I realize that sometimes (maybe always) there is some transmission and some reflection at the same time, but what determines how much of each? When a beam of red light strikes a diamond head on, and a piece of graphite head on, why will it proceed through the diamond, but only be absorbed and slightly reflected by the graphite? I realize it's something to do with different structure or configuration at the atomic level, but what exactly is the difference? What's actually happening that's different at the moment the photon/wave approaches the diamond and the graphite? I have yet to find a good visual that explains the reason one material transmits while another absorbs/reflects.
  2. jcsd
  3. Aug 21, 2011 #2
    1) Yes, those are forms of molecular kinetic energy and contribute to temperature. For C I suppose it may contribue to warming if the excited electron decays back to its ground state in such a way to emit 2 lower energy photons which then behave in an A or B sense; I'm not sure how plausible this situation is though.

    2) If the photon comes in direct contact with a nucleus then you'll get a sort of rebound effect similar to billiard balls. Look up Compton Scattering.
    You also need to realize that matter is mostly empty space. There's lots of room for the photon to just zoom right through unhindered. Or you can even apply Compton Scattering and reason that it may bounce around for a while before emerging; losing energy with each collision.

    3) I'm not really sure but perhaps the 'absorbtion' could just be the photon colliding with atoms and transferring energy kinetically, thus the photon loses energy and the transfered energy goes towards warming up whatever it is hitting. Generally the radio wave will just pass right through unhindered though.

    Someone else can answer more... Dinner time.
  4. Aug 21, 2011 #3


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    Please start by reading the FAQ subforum in the General Physics forum.

  5. Aug 21, 2011 #4


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    What makes you think our bodies don't block radio frequencies? RF has a way of going around bodies, just like ocean waves going around awash rocks. You'd need to make a meat wall to prove your point.

    And yes, there are a number of reported cases of burns from being too close to broadcast towers. It's even been discussed here on PF.
  6. Aug 21, 2011 #5
    Clever-Name: Compton scattering only applies to X- and gamma-rays, so I'm still not sure what happens with radio waves. When you say "transferring energy kinetically" do you mean that the photon's kinetic energy, upon collision, increases the translation kinetic energy of molecules within the chunk of matter? What would happen to the photon then?

    Thanks for the response!

    ZapperZ: I should have read the heat FAQ more carefully, that did help clear up part of the 1st question, you're right. Any ideas about the other questions?

    DaveC426913: The only burns I've read about are from large metallic objects near a broadcasting tower (the tower itself, guy wires, cranes, poles, pipes) acting as antennas that receive this EM energy, inducing a voltage or current in the metal. Humans coming in contact with this can get an electrical burn. Is this what you're referring to? I still haven't read anything anywhere about burns or heating of flesh simply from radio waves passing through a body.

    I followed your meat wall suggestion. My cellphone can receive FM stations, using the headphone cable as an antenna. I plugged in the headphones and set the phone to speaker mode so I could hear the FM station's music clearly. Then I wrapped up the headphone cable and placed it and the cell phone tight to my chest. I totally covered it with my hands and got my friend to put his hands on as well for good measure. The signal was more staticky but the music could still be heard. So it seems radio waves can be transmitted through a meat wall, but there's a limit.

    Anyone have any idea about #3 or #6?
  7. Aug 22, 2011 #6
    Riiiight.. I knew that -.- I should have read the source I was trying to direct you to first!

    Well, when I made that statement I was referring to Compton Scattering, where by conservation of momentum the photon 'bounces' off an atom, causing the atom to gain some kinetic energy via the change in momentum. The recoiling atom would then just cause some vibrational energy within the chunk of matter. The photon would go off on its new path with a little less energy and collide or not collide with another atom
  8. Aug 22, 2011 #7


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    Yes. Direct burning straight from radio output. Wish I could find the link. The reports say it felt like buzzing or some such.

    Man, that would have been an awkward time for someone to walk in on you two...
    [PLAIN]http://15.forumer.com/uploads/blindman/post-12-1309295974.jpg [Broken]
    Last edited by a moderator: May 5, 2017
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