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Homework Help: Oscillating E and B fields

  1. Sep 1, 2010 #1
    1. The problem statement, all variables and given/known data

    Make an order-of-magnitude estimate of the (magnitude of the) oscillating optical-frequency electric and magnetic fields in the vicinity of a light bulb.

    2. Relevant equations

    I have no idea as to what equations I should be using!!

    3. The attempt at a solution

    No idea at all! This is a hopelessly difficult task:cry:
  2. jcsd
  3. Sep 1, 2010 #2
    This is just a suggestion. I'm also really unsure about the question.

    For a common incandescent light bulb, the radiation it emits is mostly near/in visible spectrum. This is just my speculation, because if it mostly emits ultraviolet or infrared rays, it harms people and thus wouldn't be used widely. If we know the range of electromagnetic radiation that the light bulb emits the most, we get the answer to the question easily.

    Anyway, we should find some way to deduce that the frequency that the light bulb most emits is near the visible range. So now it's your turn:
    1. Taking the idea of Wien's displacement law, which gives us the wavelength that a black body emits the most (by solving [tex]\frac{\partial u}{\partial \lambda}=0[/tex]), we can find the frequency of the peak of emission from black bodies, by solving [tex]\frac{\partial u}{\partial f}=0[/tex]. Note that the peak wavelength doesn't correspond to the peak frequency.
    2. Assume that the light bulb is a black body. A common light bulb has operating temperature of about 3000K. From here, find the peak frequency of light bulb. This will give you the answer.
  4. Sep 1, 2010 #3


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    Mostly infrared I'd say. See http://en.wikipedia.org/wiki/Light_bulb#Efficiency_comparisons.
    Seems interesting. I'll investigate what is the "u" function.
  5. Sep 1, 2010 #4
    Yep. By "infrared", I mean radiation whose wavelength is very far from visible range (visible range is so narrow, so most of the time, things emit mostly radiation out of that range). My rough calculation showed that the peak wavelength is a bit above 750nm, so strictly speaking, it lies in infrared range, but saying in a "tricky way", I'll say it lies near the visible range :biggrin:

    Sorry for not clarifying it earlier. It's specific radiative intensity of black bodies (I'm not sure how it's called in English).
    And, to the OP, note that the "u" functions in those 2 equations are different.
    Last edited: Sep 1, 2010
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