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Homework Help: Electromagnetic Radiation

  1. May 28, 2008 #1
    1. The problem statement, all variables and given/known data

    If electromagnetic radiation is made up of quanta, why don't we detect the discrete packages of energy, for example, when listening to the radio?

    2. Relevant equations


    3. The attempt at a solution

    Does it have anything to do with the type of wavelengths or the lengths of the wavelengths?
  2. jcsd
  3. May 28, 2008 #2


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    What exactly do you mean by detect?
  4. May 29, 2008 #3


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    Hi clarkandlarry! :smile:

    Hint: compare with similar problems:

    Why don't we detect individual photons with our eyes?

    Why don't we detect individual electrons with an ammeter? :smile:
  5. Jun 1, 2008 #4
    So it is because the lengths of the wavelengths are so small our ears are unable to detect the packets of energy?
    and what exactly do you mean by the "types" of wavelengths?
  6. Jun 1, 2008 #5


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    Why are you asking us? You introduced that phrase in your initial post.
  7. Jun 1, 2008 #6
    im asking that because i don't know what 'types' of wavelengths there are
  8. Jun 1, 2008 #7
    Surely it has something to do with the fact that your ears are designed to detect pressure fluctuations and your eyes are designed to detect E/M waves (over a VERY limited frequency range), and the two generally don't mix? We humans can't see individual photons since it takes at least two or three photons to trigger the nerve impulse in the retina.
    Last edited: Jun 1, 2008
  9. Jun 1, 2008 #8
    There are a whole range of wavelengths for light, going from the very small [tex]\lambda=(roughly)\ 10^{-8}m[/tex] to the very large [tex]\lambda=1m[/tex] all with a corresponding frequency and energy. The same is true for sound waves, although the relationship between frequency wavelength and energy is not as straight forward.
  10. Jun 1, 2008 #9


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    The wavelength of electromagnetic waves could theoretically be any length whatsoever, i.e. anything between 0 meters and infinity meters. For radio waves, it will be in a narrower range corresponding to radio wave frequencies.

    But that's not really relevant to this problem.

    This has more to do with how often individual radio-wave quanta are received by a typical radio.
  11. Jun 1, 2008 #10


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    Ask yourself this question: just how many photons are reaching your radio every second when you listen to your favorite station? You have an equation for the energy of a single photon in terms of its frequency. So pick a convenient radio frequency, say, 100 MHz (i.e., "100 on your FM dial") and calculate the energy of a single photon.

    Now consider the power of the radio signal being sent out. For a typical commercial station, this can easily be 10 kW. So how many 100-MHz radio photons is the station's antenna emitting every second?

    Pick some reasonable distance of your receiver from the antenna, for instance, 10 km. Assume the radiation of this photons is distributed equally in all directions ("isotropic emission") and figure out how many photons are crossing a 1 square meter area on a 10 km. sphere each second. (This is the "intensity" of the radio signal at your receiver). For convenience, you could take this as the number of radio photons you are intercepting every second. Does one photon more or less matter much compared to that number? If it does, you can "detect" the individual quanta of radio energy. If not,...
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