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Homework Help: Yet ANOTHER frequency-amplitude question (sorry guys)

  1. Jan 31, 2010 #1
    Okay, so I'm generally a pretty smart guy, always been a science type and I like teaching / helping others. My girlfriend is in a university astronomy course and she was studying the electromagnetic spectrum and made a claim regarding fequency and amplitude that I quickly disagreed with, now as I care about her, I'm just trying to make sure that I'm not steering her wrong here and I can't seem to find the answer anywhere online or in previous questions, so hopefully you guys can help me!

    What she told me:
    "the amplitude of a light is given by the brightness of the light, the brightness of the light varies as photons / second, therefore, at higher frequency wavelengths, more photons / second will be observed and therefore one can assume that the amplitude of the wavelength varies as frequency"

    Now that doesn't make a damned iota of sense to me, I told her that frequency and amplitude are completely unrelated (at least, perhaps from a natural source) but I drive a forklift and it's been 5 years since I've been in school so I'm really not sure anymore :( - however the logic of her statement (which she says she copied out of a text book and not from her professor's notes) seems infallably valid, so all I can think is that photons don't have much to do with wavelength and therefore are themselves unrelated to frequency -- getting rid of that pesky correlation between frequency and amplitude.. I'm just really not sure these days!! It's just been too long I guess lol.

    Can someone describe the relationship between amplitude and frequency for me? As it relates to the electromagnetic spectrum? (which I'm assuming, if I'm wrong, is a different, and viably so, relationship than they have when we're talking about sound waves)

    Thanks a lot guys!
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Feb 1, 2010 #2
    Amplitude does correspond to number of photons per second but wavelength and frequency correspond to the color of light or equivalently the energy of each photon. So if you were to find a light source whose color changes as it gets brighter, then frequency would be related to amplitude.

    This is confusing/ambiguous. "Amplitude" is not a word commonly used to describe wavelength, and wavelength is related to frequency (frequency = speed/wavelength)

    There are cases where frequency does change with amplitude. In fact, I believe most real systems are not perfect harmonic oscillators. For instance, a pendulum is often modeled as a harmonic oscillator, but if you were to start a pendulum at two different heights you would find that it begins swinging at two different frequencies. I have a feeling that light--or rather light sources--may behave the same way at high intensities, but I haven't studied enough physics to know for sure (and I doubt that's what your girlfriend's textbook was referring to)
     
  4. Feb 1, 2010 #3
    Yeah see that's what I was thinking.. so then a photon doesn't correspond to a single wavelength? (which makes sense as the wavelength of the light should be measurable more by the effect of the photon passing through the atmosphere or water / some sort of translucent substance I would think, rather than it being a measure of specific photons or photons / second correct?)

    To be clear, I want to confirm:
    .......^..............................
    ...../....\............................
    .../........\......................(+)
    _/_______\_______________.....DOES NOT EQUAL.....(1 photon)
    ................\.........../..........
    ..................\......./........(-)
    ....................\.../..............
    ......................v...............

    (where 2 whole waves would equal 2 photons etc.)

    regardless of frequency, wavelength or amplitude.

    Thanks again for your help guys! :)
    Regards,
    A
     
    Last edited: Feb 1, 2010
  5. Feb 1, 2010 #4

    Redbelly98

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    You're correct that one wavelength does not correspond to one photon.
     
  6. Feb 1, 2010 #5
    Perfect, thanks :)
     
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