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B Light bulbs -- is there a continuous wave spreading out?

  1. Aug 8, 2017 #1
    When a light bulb is emitinting light, is there a continuous wave spreading out, or is there a large number of particles (photons) emitted in random directions, which in the conglomerate, mimics a single continuous wave spreading out.
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  3. Aug 8, 2017 #2

    Vanadium 50

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    It's many waves of many wavelengths.
  4. Aug 8, 2017 #3
    That is not what I asked. To simplify the matter, assume a momochromatic light source.

    However, assuming that you will again reply with a wave, how is it that a filliment, which may be more than an inch long, emit a wave from one end to the other, especially when the wave length is considerably shorter than the length of the filliment?
  5. Aug 8, 2017 #4


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    Part of what you don't understand is that photons are not like bullets. Bullets flying in the air don't intereact with other bullets. Multiple photons traveling together interact with the nearby photons, making the whole group act more like a single wave rather than a collection of independent particles.

    Where photons are involved, neither "wave" nor "particles" tells 100% of the story. Quantum mechanics is weird, common sense and everyday experience can't explain it.
  6. Aug 8, 2017 #5
    Really it's noise - the best analogy would be listening to static - it does revel through the air as a wave but it is a random signal. Random emissions, (photos) but at random wavelengths. Not a continuous waveform and not a single wave, and it does not have anything to do with the length of the filament.
  7. Aug 8, 2017 #6
    "Number of photons" is not always definable in QED. I.e. whether you can define proper "number operator" depends on the system you are considering. As far as I remember, light bulb is one of the systems where you can't talk about number of photons. There are plenty of threads about those issues, for example:
  8. Aug 8, 2017 #7
    How would you describe, from the point of view of quantum mechanics, at the atomic level, the process that causes an incandescent light bulb to emit electromagnetic radiation, i.e. light? Where does the radiation come from and how is it emitted?
  9. Aug 8, 2017 #8


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    That's a fair description and they will never 'add up' to produce a single wave.
    If the source of light is a hot body, it is a vast number of totally unrelated little radiators and there will be no coherence at all. Each photon that's emitted will be totally unrelated to all the others. Put it another way, you will have a vast number of waves (wavelets, if you like) which will end up, totally independently at a detector of some sort. If the source is an ordinary monochromatic discharge tube, the situation will not be much different except that diffraction and interference effects will be a bit more visible. Your 'monochromatic' source will still have a significant range of frequencies and again each atomic emitter will be independent so what comes out will not be basically coherent. It will only behave like 'a wave' under very simple circumstances and you would need to collimate it through a slot. If you have a laser source, the coherence is much better and the photons were produced more or less 'in phase' so you can almost say you have 'a wave'. However, even a laser will have a finite bandwidth and its coherence length will not be infinite. The reason that you need a laser for holography is that the lengths of the paths that the light has to take are so different that only laser light is coherent enough to behave truly as 'a wave'.
    BUT Note. When you describe how lenses, mirrors and most other optics work, you CAN treat totally incoherent light as a wave. (It's near enough for those situations.)

    Not sure about that. Aren't photons Bosons? That should mean that they don't interact with each other, shouldn't it? It's the action of a laser that makes all the photons produced act 'as one'. That's due to the interactions between the photons already in there and the atoms that are waiting to emit their photon (Stimulated emission) in phase with the ones going past.
  10. Aug 8, 2017 #9


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    You don't need Quantum Mechanics to answer your question and it just confuses the issue.

    The radiation comes from the heat in the filament. The current you supply to the lightbulb conducts through the metal of the filament. This current generates heat that must be dissipated. This dissipation is due to electromagnetic radiation whose wavelength depends on the temperature of the metal.

    For more detail you can see, for instance, https://en.wikipedia.org/wiki/Thermal_radiation
  11. Aug 8, 2017 #10


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    Actually, if you don't use QM to describe black body radiation, you get the totally wrong answer. Classical analysis gives you The Ultraviolet Catastrophe Planck solved the problem but only by introducing Quantum Mechanics. :wink:
  12. Aug 8, 2017 #11


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    Touche, but we're just talking about how a lightbulb works... :wink:

    "Daddy, how does a computer work?"

    "Well son, first you start with Pauli's Exclusion Principle..."

  13. Aug 8, 2017 #12


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    No problem there - except the things that radiate the photons (which have already been introduced into the thread) are individual atoms and molecules. The spectrum of the emitted EM is the sum total of all of them and not "a wave".
  14. Aug 12, 2017 #13
    I began this thread, primarily as a fishing expedition. However, your post brings up a rather common behavior on forums. Regardless of the forum, regardless of subject matter, or the supposed intellectual level of the respondents, the greater part of replies tend to fall along either bumber sticker type of lines or those that dismiss the posts, the ideas, of others without ever providing better ideas. Apparently it is easier to laugh at others, anonymous others, than admit what one does not know one's self.

    By contrast sophiecentaur gave a thoughtful response that is very much in line with what is happening, on the atomic level, when incandescent light bulbs are emitting light.
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