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Light/wave travel/propogation

  1. Aug 9, 2009 #1
    Can someone explain to me why waves do not "affect" an object that is shorter than its wavelength?
    For example, a popcorn kernel cannot be heated by a photon with a wavelength larger than the size of the kernel, or a sound from a tuba cannot be heard when one is standing in front of it unless the sound coming out from it travels a distance of at least one of its wavelength (sound travels maybe 4 feet in tuba but the wavelength of the sound is 8 feet so the listener would have to stand 4 feet away from the tuba in order to hear the sound).

    I'll elaborate more if you need me to.
     
  2. jcsd
  3. Aug 9, 2009 #2

    turin

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    I can offer a simple counter example to the basic claim:
    A 3-meter ocean wave makes a 1-meter buoy bob up an down in the water. So I would say that the ocean waves "affect" the buoy. I think that the claim is too broad. It is at least too vague to comment on.

    Regarding your specific examples:

    You can get an idea for an asolute minimum bound on the wavelength of the microwaves by examining that mesh pattern on the window of the oven. Those holes had better be smaller than a wavelength, and much smaller is better. As far as I'm aware, the wavelength of microwaves in a microwave oven is roughly 10 cm. I am not aware of any popcorn kernels that are anywhere near that size. I often enjoy popping corn in my microwave. So, I believe your specific claim about the popcorn kernel to be false. In fact, the heating that occurs in a microwave oven is analogous to the bobbing buoy, as it is dielectric heating, which uses an effect of the dipole approximation, which only requires a changing electric field in time at a sufficient rate.

    I am not familiar with this tuba effect, but I am skeptical. I don't know where I'm gonna find a tuba player to demonstrate this for me.
     
  4. Aug 9, 2009 #3

    diazona

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    I highly doubt the tuba effect as well. For one thing, if it were true, how would the tuba player hear his/her own instrument? And although I can't remember a specific instance, I have a pretty good feeling that I've listened to a tuba from closer than 4 feet. (If it comes to that, I know about half a dozen tuba players I could ask :wink:)
     
  5. Aug 10, 2009 #4
    Hmm I must've heard it wrong or the people I heard it from had no idea what they were talking about. I thought about them and the popcorn one probably had to do with the fact that the energy of a photon is proportional to its frequency, so a large wavelength would have a low frequency and therefore low energy - insufficient for the water inside the kernel to boil. I'm not sure about the tuba effect, if anyone can test this out or even play an electronic sound with a long wavelength and report back, I'd appreciate it.

    One example that I'm sure of however is that microwave ovens have spinning plates because the microwaves "skip" areas (depending on their wavelengths I believe). If there wasn't a spinning plate, your food would be cold in some spots and warm in others. Can someone explain this? I also thought of another example involving the Heisenberg uncertainty principle but I couldn't put it into words but if you know or have an idea of what I'm thinking about, please tell.
     
  6. Aug 10, 2009 #5

    diazona

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    I tried it with a 110 Hz sine tone (3.1m wavelength) and I was able to hear it just fine with my ears only a few inches from the speaker. Plus it flies in the face of everything I've heard in my 10+ years of playing in bands. So I'm positive the "tuba effect" is bogus.
    Yeah, basically you have a standing wave pattern set up in the interior of the microwave. If you're not familiar with standing waves, the important thing to know is that certain points are nodes, where there is no oscillation of the wave, and other points are antinodes, where there is the maximum oscillation. If the food didn't rotate, it would make sense that any food at the antinodes wouldn't get heated.
    No idea... there are many, many applications of the uncertainty principle, I have no idea which one you might have in mind.
     
  7. Aug 10, 2009 #6

    turin

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    Consider that a distinct possibility. It reminds me of so many things that adults tell children in order to avoid a hassle (for many examples, see Calvin and Hobbes in which Calvin's dad "explains" things to Calvin). For instance: "All of the vitamins are in the skin." Ha. Sad to say, I only very recently realized that one. (I now peel the carrots potatos when we have guests, but I still keep the skin on when it's just me and the misses.)

    Probably shmobably. Are you telling us that you've never popped popcorn in a microwave oven??? The beauty of science (scientific thinking) is that someone can reason all they want, but they are not allowed to argue with the experimental observations. (Well, interpretation of the observations, maybe, but popped popcorn is irrevocably distinct from unpopped popcorn.) How does the person making the claim then explain the popping of the popcorn in a microwave oven, if the microwaves are "too long"?

    Yes. The microwaves are injected from a source; they do not magically appear everywhere inside the oven. In fact, they are generated resonantly and coherently, but they are being used for a very incoherent purpose: cooking. They do manage to bounce off of the walls of the oven, but the nature of the injection simply leaves some spots "cold". It is similar, although not as extreme, as sunbathing. If you lie on only one side, your other side will not get tan. You must turn in order to distribute the tan evenly.

    You could be referring to several issues. I will guess: You cannot measure the position of a particle more accurately than about the wavelength of the light that you use to probe the position (one of Heisenberg's original conceptions). There are many other consequences of the principle as well, and in fact they are all based on a limit imposed by wavelength in some way or other.
     
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