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I Are continous Spectra actually discrete?

  1. Apr 22, 2017 #1
    If we assume that there is a fixed charged particle and another charged particle is spiraling down towards it, it emits electromagnetic waves as it is accelerated motion. We get a continuous spectrum. Now, if I allow the emitted photons to fall on a very photosensitive material, whose efficiency is known, then i can get photo electrons for those photons whose frequency is beyond threshold frequency. By knowing the current, i can know the number of electrons and from number of electrons, i can calculate the number of incident photons by dividing the number of electrons by efficiency. As photons have a unique frequency, i can determine the number of frequencies the photon can assume between the range in which it can make an electron come out. So this means that i am getting discrete values. But the spectrum is continuous everywhere. So, that's my question. Where am I making a mistake?
  2. jcsd
  3. Apr 22, 2017 #2
    The "continuous spectrum" statement relates to a vast number of photons taken together. If you plot enough of their energies, you will get a smooth curve. But as you say, each photon on its own is just a data point in that plot with a distinct frequency and energy.
  4. Apr 22, 2017 #3
    I meant that if the spectrum is continuous, then I can get an infinite number of photons in any range of frequency. Let the frequencies in this range be more that the threshold frequency. As photoelectric effect is one photon one electron phenomenon, i must get infinite number of photo electrons (we make sure that the photo sensitive material remains neutral.) Also efficiency cannot be zero. So zero x infinity thing doesn't work. So that is where the problem is.
  5. Apr 22, 2017 #4
    Continuous spectrum does not mean that at any given time, all frequencies are present. That would indeed mean infinite number of photons, which obviously isn't true.
    Continuous here means that the photons that *do* get emitted can have any frequency in that spectrum.
  6. Apr 23, 2017 #5
    So actually the spectrum that I get from a charge spiraling inward is a line spectrum with the lines very close to each other. Will this solution work out?
  7. Apr 28, 2017 #6


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    You'll get a finite number of photons but I wouldn't call this a "line spectrum" with the lines very close to one another. To me that implies a line spectrum implies spectral lines at certain energies, such as the spectral emission lines of a gases. If you were to perform this experiment over and over you would get photons of all frequencies from some lower range to some upper range.
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