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Standing waves in a microwave oven

  1. Apr 26, 2007 #1
    Has anyone out there actually tried the kitchen experiment of placing a continous line of meltable food (i.e. marshmellows/chocholate/cheese etc.) across the bottom of a microwave oven in hopes of finding the antinodes of a standing wave by looking for regions that melt while the rest of the line remains solid? I have, and while I did find random schorch marks, there was by no means a regular pattern of them (as you would hope from a standing wave). It seems to me that the experiment is doomed since the incoming waves are "bounced" first off a rotating fan blade to somewhat "randomize" the waves so that there are not going to nodes and antinodes. What puzzels me is that I still see this experiment on the Web as a way to determine the speed of light from this kitchen appliance. Am I missing something?
  2. jcsd
  3. Apr 26, 2007 #2
    Not all ovens use a rotating reflecting thing to randomize nodes. Some use a turning table (to turn the meat). In mine, there is a switch that allows stop the table turning.
  4. Apr 26, 2007 #3
    lpfr, I'm not an expert but I believe that they do all have the defecting fan in addition to the rotating table.
  5. Apr 26, 2007 #4
    Neither me. But I know that in catalogs you have (at least had) the choice between models with each system.
    I have opened mine (longtime ago) and I assure you that it do not has a turning deflector.
  6. Apr 26, 2007 #5
    Well then, your microwave oven would be ideal to check out how well you can detect the antinodes. The spacing should be ~6.5 cm (frequency=2.45 GHz). I cut out a piece of cardboard to fit the inside cavity and then completely covered it with thin slices of American cheese. As I said, I got just random burn marks but from yours I would expect burn "stripes" equally spaced. I suppose if the direction of the microwaves is not parallel to the cardboard then lamda/2 (~6.5 cm) need not be the spacing. In any event, do you have plans to run the experiment?
  7. Apr 26, 2007 #6
    I have to obtain my wife's permission. It is her oven! But maybe...
  8. Apr 26, 2007 #7
    I'll be waiting for results with bated breath
  9. Apr 26, 2007 #8
    lpfr, I just realized that we traded a few responses a few weeks ago regarding reflections of light from smooth metal surfaces. I was attempting a quantum explanation in terms of photons; you had a classical E/M explanation. It seems to me that both approaches have to work, but if the light intensity is low enough then you have to consider the "beam" of light as composed as individual photons. Any thoughts on that approach?
  10. Apr 26, 2007 #9
    I just did the experience. I just heated a wet cardboard and observed the places where it dried faster. The pattern of dry spots was not as clear and beautiful as I hoped. But the distance between dry spots was about 6-7 cm. Assuming that this is half a wavelength, this gives a frequency of 2.3 GHz which is a good value (and far better than my expectations).
  11. Apr 26, 2007 #10
    Wet cardboard, thats a good idea. Could you describe the "pattern", were there stripes, or was it linear array of spots? When I did it I was looking for the 6-7 cm spacing and I found it, but I doubt it would have convinced a skeptic
  12. Apr 26, 2007 #11
    It is not easy to explain why a photon behaves in the same way as a wave. You have to say that the photon behaves as if it takes all possible paths. But this assumption is not part of quantum mechanics postulates. It is a "parachuted" assumption to explain the unexplainable.

    I'm no good in quantum mechanics. It hurts my intuition. Then I do not try to explain the entire world in quantum mechanics terms. I use QM only when classical mechanics don't work. This is usually the way physicist work. They take each time the simplest model that explains a phenomenon. May be it is commodity or lack of masochism or, as in my case, ignorance.
  13. Apr 26, 2007 #12
    I would have liked a checkers board. But no, there where stripes, but they do not have all the height (the cardboard was vertical about the center of the oven). And they where not regular over the surface.
    But there are hundreds of modes in a cavity far greater than wavelength and the one I saw is not the prettiest.
  14. Apr 26, 2007 #13
    Years ago I was using an electron microscope (transmission type) when I realized that the current of the electron beam was such that electrons were traveling through the sample one at a time. This makes it difficult to explain electron diffraction (which I was observing quite well) since there was only one electron (at a time) to cause an interference pattern. The only explanation was that the electron was interfering with its own "propablility wave" which was being scattered by the regular array of atoms in the crystalline sample. Ever since then I have always tried to explain things from a individual photon point a view. Sometimes its difficult though. Signing off for now.
  15. Apr 27, 2007 #14
    There's no need to describe microwaves quantum mechanically. All of their relevant properties can be gotten straight from Maxwell's equations.
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