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Microwave Vs. Visible light Heating

  1. Jun 13, 2014 #1
    People always tell me that microwave ovens heat food by exciting the water molecules in food. This is done by blasting food with 2.4 Ghz radiation, the people that say this, also say that the frequency that is used is critical because that frequency interacts with the water molecules. I wonder why then we use that frequency for wireless communications, for instance wifi. If it heats our tissues.

    My major question is how does a visible light laser heat tissue if it uses a very different frequency? Also, can exposure to non ionizing radiation cause a atoms electron to be dislodged making the atom a ion, leading to cancer?
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  3. Jun 13, 2014 #2


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    It's not so simple as frequency = X means Y molecule will absorb it and become excited. Molecular absorption lines are very different than atomic absorption lines. There tend to be many of them, and, depending on the complexity of the molecule, there can even be a rough continuum of absorptions. So the physics is pretty complicated.

    But, generally speaking, water molecules tend to absorb at those frequencies (e.g. 2.4GHz) better. That certainly isn't the ONLY frequency that water molecules will absorb though.

    As far as wifi signals vs microwave ovens. A microwave oven generates standing waves inside which basically uses constructive interference to amplify itself so that it can heat up the objects inside. In addition, a microwave is MUCH higher power than a wifi router (wifi routers don't typically take 1200 watts), and it's waves are enclosed in a small space. A wifi source has a much much weaker power, and is spread over a much larger area so it won't "cook us from the inside".

    Visible light is definitely absorbed by tissue. Can you see through your skin an muscle? You can't. This means that visible light is being absorbed all the time. You might notice bright sunny day, your skin starts to burn after a while. That's your skin absorbing the light from the sun.

    A laser just focuses this energy into a very small area so it can do damage.

    Lastly, non-ionizing radiation is defined as such that it doesn't break up atoms (it doesn't "ionize" them), so non-ionizing radiation is much less dangerous. However, in high enough doses, it might become dangerous. You probably don't want to stand right next to a radio tower's big antenna for very long, for example.
  4. Jun 13, 2014 #3


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    There's nothing really special about 2.4ghz and heating food (that contains water) in a microwave oven other than the penetration depth cooks food fairly evenly at that wavelength, it's easy to shield/concentrate and it's in the ISM (Industrial, Scientific and Medical) band. Higher frequencies (10-100GHz) would be actually be absorbed better by water but would burn the surface of food before the middle was cooked.
  5. Jun 13, 2014 #4
    When my skin absorbs visible light, what is actually absorbing the photons, the nucleus, electrons, or both? Is it true that when the nucleus is put into a high energy state that it will dump energy as electromagnetic radiation? What is causing us to emit infrared light, the energy is to high in the atom and it needs to lose some?
  6. Jun 13, 2014 #5


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    Most of the absorption and re-emission will be from molecules, not individual atoms, since you're made of molecules and such, not individual atoms!
  7. Jun 13, 2014 #6

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    That the 2.4 to 2.5 GHz band is one of the ISM bands precisely because of microwave ovens. Microwave ovens standardized on 2.45 GHz well before the frequencies around 2.45 GhZ were designated as one of the ISM bands. When the FCC and its international equivalents decided they needed to carve up the radio frequencies into regulated and unregulated chunks it would have been downright stupid to turn microwave ovens into regulated devices. They instead made that frequency band that surrounds that commonly used microwave oven frequency into an unregulated band.

    Electromagnetic radiation is photons and nothing else. Low frequency radiation (up to low ultraviolet) is non-ionizing. High frequency radiation (high UV, X-rays, and lower frequency gammas) knocks electrons from atoms and molecules. So what? What's a lost electron or two? The problem is that this disrupts the complex molecules in your body. The charged molecules function differently, sometimes disintegrating, other times recombining. Ionizing radiation seriously messes with life at the biochemical level.

    High frequency gammas are even worse. They can hit your nuclei.

    What about lower frequencies? That depends a lot on intensity. While microwaves are not ionizing radiation, you do not want to put your cat in the microwave. Ripping electrons from molecules isn't good for life. Neither is getting those molecules vibrating so wildly that they become cooked.

    The difference between ionizing versus non-ionizing radiation is that ionizing radiation does minor damage even in small doses. Non-ionizing radiation in small doses is like a tiny puff of slightly warm air.
  8. Jun 13, 2014 #7


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    You could cook some things with visible light, but not water, since it's transparent, and only the surface of most meats and vegetables, since light won't penetrate very far (maybe a few millimeters) in most foods. Overall, not very useful compared to microwaves. That said, heat lamps are often used to keep catered foods warm, and that uses infrared and visible light.

    Nonionizing radiation won't cause electrons to be dislodged. It will just cook you. But only if you are exposed to enough to really raise your temperature. You will feel it.
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