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Does microwave radiation linger inside a microwave oven?

  1. Jan 21, 2009 #1
    I was curious about microwave ovens and how they work and in particular about the microwave radiation they emit and the halflife of this radiation. My layman's understanding is that microwave ovens use a 2.43GHZ emf to heat up water inside food items by attenuating 1 or more? of the few absorbtion bands that water has. What occurs right after the microwave is done cooking a food item and it turns off? Does the microwave radiation instantly dissipate, or become absorbed by the inside walls? Also, the microwave doors are fine meshes with holes roughly, from my estimation, perhaps 5 mm in diameter so my question is what is the largest diamter these mesh holes could be, whilst still stopping radiation leakage from the microwave. Conversely, what is the relationship between the containing the radiation and hole size? And what about the materials used to shield microwaves, do they use lead, copper, gold, other? And what about starting sentences with the word 'and'? Are average people allowed use such poetic license, or must you be a
    William Shakespeare or a William Faulkner? :smile:
     
  2. jcsd
  3. Jan 21, 2009 #2

    russ_watters

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    Microwave radiation is like light: it absorbs quickly into objects after the source is turned off. The words "radiation" and "radioactive" are not necessarily applicable at the same time. Light and many other forms of EM radiation, including microwaves, can be emitted by non-radioactive sources. And radioactive isotopes have half-lives, the radiation they emit does not. It's just like light.

    For shielding, you can see it: it's the metal screen behind the window. An EM radiation shield, in general, merely needs to be a metal screen with smaller holes than (half?) the wavelength of the radiation to be reflected. You might notice satellite dishes have similar screens, with larger holes for longer wavelengths.
     
  4. Jan 21, 2009 #3
    What russ said. It's like asking what happens when you turn off your flashlight. "It stops."
     
  5. Jan 21, 2009 #4
    I guess the holes can be thought of as diffraction gratings, right? From what I think I know about radio communication, your antenna has to be 1/2 the wavelength or bigger, to attenuate an incoming transmission. Since radio waves and microwaves are both emf radiation, I am thinking your 'half?' guess is correct as well.

    2.43 GHZ equals a wavelegth of 12.3 cm, clearly this is much larger than the hole size of 0.5 cm found on my microwave's mesh screen, 24x larger in fact. So even if we were corrent on the 'half' wavelength theory, one can still see my microwave's holes are much smaller than they need to be. This makes me wonder if the mesh screen gives added protection around our 2.43 GHZ operating frequency, maybe 2.43 GHZ is the peak of the microwave's emission spectra and less intense radiation is emmited on either end of this 2.43 GHZ amplitudal peak? Maybe we are being given added protection since 0.5 cm corresponds to 60 GHZ.
     
    Last edited: Jan 21, 2009
  6. May 30, 2009 #5
    The holes in a microwave oven door window represent a phenomenon known as a "waveguide beyond cutoff". The holes are extremely small so the waves can't get through since the free space wavelength is 12.2 cm. However, they can bulge into the hole so there is a relationship between diameter, or rectangular dimensions, and thickness. We have built a "window" in the side of a research microwave oven that is an aluminum tube 1" x 1" and 6 inches long. The microwave field bulges into it by over a centimeter, but at the viewing end there is zero microwave energy.
     
  7. Nov 1, 2011 #6
    russ_waters said: "Microwave radiation is like light: it absorbs quickly into objects after the source is turned off."
    But if I have understood correctly, the walls of the microwave oven almost completely reflect microwave radiation. So the equivalent thing would be would a light source within a room of mirrors.
    So why does it say everywhere that there no radiation escapes when the oven is opened? Say that we had an empty oven and we open it, wouldn´t the radiation that had not been absorbed by walls and magnetron escape through the open door?
    Hope someone is still there!
     
  8. Nov 1, 2011 #7

    Vanadium 50

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    Considering the last post was more than two years ago, probably not.
     
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