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Basic physics question (frequency/energy relation)

  1. Sep 1, 2015 #1
    I almost feel embarrassed to ask this. I've always understood it to be that the higher the frequency, the higher the energy (at least from what I've been taught). However, something has always bugged me: why then, when a medium is involved, are the lower frequencies the ones that make it through? Wouldn't this mean that the lower frequencies have more energy to perturb that medium?

    Examples of what I mean,
    Sound: If someone is blasting music in their car, or you walk outside past a night club, you hear the bass (lower frequency).

    Light: At sunrise/sunset, the sun looks yellow/gold because it has to travel through more atmosphere, deflecting the higher frequencies and only the lower frequencies (orange, yellow) reach you eyes.

    Perhaps this has to do with resonant frequencies but I'm not sure. Could someone explain the relationship between frequency and energy that explains this?
  2. jcsd
  3. Sep 1, 2015 #2


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    Staff: Mentor

    Consider water waves. It's easy to build a breakwater that will protect a harbor against the highly energetic waves that a storm drives in - these smash excitingly and energetically against the breakwater, throwing spouts of white water many meters into the sky with every peak in amplitiude. But compare with the low-energy, low frequency (twelve hours!), long wavelength (halfway around the earth!) wave represented by the tides.... these drift gently in, passing the breakwater as quietly as they pass any other random piece of topography along the coast, and over a period of hours they move the water level in the harbor by their full amplitude.

    Intuitively, what's going on here is that the lower-frequency wave is changing the height of the water more slowly so that the water level has time to adjust and the next increase in height will build on the previous one. The higher-frequency storm wave piles water up against the breakwater, but before the water has time to flow on into the harbor the next trough has come along.
  4. Sep 2, 2015 #3
    First, your statement that "the lower frequencies are the the ones that makes it through" is a generalization without support.
    Metals are opaque to visible but transparent in UV and some bands in x-rays. Just one example that contradict this statement.
    Transmission coefficient of EM waves is a complicated function of wavelengths and you cannot make any general statements, valid for any material and spectral range.
    Air scatters more blues than red due to the size of the scattering centers rather than the energy of the radiation. It is valid when the model of Rayleigh scattering is applicable.
    You can have a medium that scatters more lower frequencies than higher frequencies.

    And second, the statement that higher frequency waves have more energy than lower frequency waves makes sense for quantised waves, like the EM waves. It just means that a quanta has more energy at higher frequencies.
    In the case of sound waves described by a continuous model, what would this mean? A tsunami wave with amplitude of tens of meters will have less energy than an ultrasound beam used for medical imaging? The energy of the wave itself depends on both amplitude and frequency.
  5. Sep 2, 2015 #4


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    If you refer to the equation
    E =hf,
    that relates the energy of photons to their frequency. It doesn't relate to the amplitude of a wave, which (squared) relates to the Power density. The number of photons for a given power will relate to individual photon energy.
    Linking this to the behaviour of mechanical (water) waves is a big step. Is all that not more related to matching of the different wavelengths to the structures involved and propagation losses of surface water waves?
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