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Compact fluorescent light bulb in microwave

  1. Jul 7, 2010 #1
    So when you watch those youtube videos and they put a compact fluorescent light bulb
    in the microwave and it starts to glow , but how come we get visible light from something that we are shooting microwaves at , it seems like conservation of energy does not hold .
    but then when i was reading about florescence , it said that under extreme intensity the electron can absorb 2 photons and then kick it up to the next energy level , so is this what is going on in the microwave with the light bulb . And when you learn about the photo-electric
    effect intensity doesn't matter right ? or is this to a point , And can an electron absorb more than 2 photons .
     
  2. jcsd
  3. Jul 7, 2010 #2

    DaveC426913

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    Gold Member

    It is the phosphorescent coating on the inside of the glass that absorbs and then emits visible light.
     
  4. Jul 7, 2010 #3
    ya but i thought that coating took a higher energy photon and then turned it into visible light . like uv rays into visible light
     
  5. Jul 7, 2010 #4

    alxm

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    I don't believe the coating is phosphorescent. At least I haven't noticed it glowing for any significant period of time after turning the light off. And where would the visible light it's re-emitting be coming from?


    If you put a piece of metal in a microwave, the microwaves induce a charge separation and the potential difference is big enough to cause arcing. Put a fluorescent bulb in there and you will induce a potential difference across the bulb, which is a device meant to arc. So it arcs and gives off light the same way it does otherwise, by running an ionizing current through the gas.

    This is correct AFAIK.
     
  6. Jul 8, 2010 #5
    The microwaves make the lamp glow producing uv which excites the phosphor. The microwaves don't have enough energy to excite the phosphor directly.
     
  7. Jul 8, 2010 #6
    The microwaves have enough energy in total - they just don't have the energy-per-photon that's needed for a direct excitation. But the lamp design gets around that by converting the microwaves to UV.

    You're just supplying the lamp with electrical energy - just what it needs to work. It'll take any sort of electrical energy it can get - it ain't fussy.
    You can get the same effect with a high voltage supply like a Tesla coil. Wave it in the air near the coil and it lights up like a light sabre.
     
    Last edited: Jul 8, 2010
  8. Jul 8, 2010 #7
    In this case are we lighting up the bulb with the electric field .
     
  9. Jul 8, 2010 #8
    Ok so when the microwaves hit the argon in the tube or the mercury they eject electrons causing charge separation , and that electron could absorb another microwave giving it more kinetic energy and when the electron gets enough kinetic energy to excite the mercury's electron up the the next energy level and then release a photon in the uv range and then that photon hits the phosphor coating and then gets absorbed and then re-emitted as a visible photon is this what is going on ?
     
  10. Jul 8, 2010 #9
    The E field is high enough to ionize the gas and allow current to flow. This is the same whether it's microwaves or the lamp ballast running at 40khz.

    The second half of your paragraph is correct.
     
    Last edited: Jul 8, 2010
  11. Jul 8, 2010 #10
    So its not the photoelectric effect from the microwaves that is causing electrons to flow inside the bulb , what is creating the E field in the microwave.
     
  12. Jul 9, 2010 #11

    alxm

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    Maybe we need to start over. Okay, how does a fluorescent bulb work to begin with?

    You have two electrodes on each side of a tube with some low-pressure gas in it. If you have a big enough voltage difference on the two electrodes, you get arcing across the gas. What happens in arcing is that you get a 'path' of ionized gas through the tube, and the current flows through this. The ionized gas, in turn, is continuously becoming de-ionized as the electrons return to their nuclei, giving off radiation in the process, mostly in the UV. So you have a fluorescent coating on the inside of the glass to pick up that UV and re-emit it as nice white light.

    Now, if you put a piece of metal (a conductor) in a microwave, it will act as a dipole antenna, and you'll get an electrical potential difference across the piece of metal. In a microwave oven, this potential difference is big enough to cause arcing, even through the air.

    So, when you put a fluorescent bulb in the microwave, you get a potential difference between the two electrodes - one which is large enough to cause arcing across the fluorescent tube. Which shouldn't come as a big surprise, since it's meant to be easily ionized. In short you've just changed the means by which you create this (resonating) potential difference across the bulb. Normally it's created by a capacitor and transformer, but here it's induced by the microwaves.

    The microwave radiation isn't absorbed to any significant extent by the gas in the tube itself; as the gases in a fluorescent tube have no dipole moment.
     
  13. Jul 11, 2010 #12
    ok i see now , thanks to everyone for their response , Maybe i should start a new thread or not , so the microwave door with a window in it , this window can let visible light through but stop microwaves , seems weird cause radio waves can go through walls but visible light cannot , I guess that window is designed in such a way that it cant absorb and re-emit microwaves based on the coulomb interaction between the electrons.
     
  14. Jul 11, 2010 #13
    I believe it works like a faraday cage: Any holes in the microwave window are smaller than the wavelength of the micro waves (which is probably around 12 or 13cm). This is because the window has a piece of metal over it with lots of little holes in it.
     
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