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Sound and Light Waves

  1. Mar 4, 2014 #1
    So, sound is a longitudinal wave, while light is a transverse wave. The speed of sound varies going from one medium to another directly proportional to the bulk modulus and inversely proportional to the density of the new medium (i.e. from air to a steel pipe). Let's say a sound wave (consisting of energy) hit a steel pipe from air, how would the sound transfer (on an atomic level) from the air to the steel pipe and then, finally, travel along the steel pipe for some distance (i.e. 30 feet along the steel pipe from where it transferred to the pipe initially)?

    Now I know that the bulk modulus (b.m = stress/strain) of the steel pipe is higher than air, meaning that it "stronger" / undergoes less strain, or less deformation with an equal force being present (for both the air and steel). My second question is, why would less deformation to occur in order for the speed of sound to go faster? My thought process is when a sound travels, there will be oscillations that occur in the direction it is traveling. Why less deformation cause sound to better transfer those oscillations (i.e. thinking about the atoms that make up steel)?

    For light, why would the opposite occur, where n = c/v? Meaning if you go to a denser medium, why would the light wave become slower? In addition, just for clarification, why would energy and frequency stay constant for both types of waves, regardless of the medium?
  2. jcsd
  3. Mar 5, 2014 #2

    Simon Bridge

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    That is not all there is to it - sound is a pressure wave in some medium while light is an electromagnetic wave that does not require a medium.

    Crudely? Air particles bang into the steel particles and set them moving.

    It is because there is bigger deformation that the wave travels slower.
    The particles have to travel further to have the same effect on their neighbors.
    The wave is like the baton in a relay race - the further the runners have to go to pass the baton on, the slower the baton moves.

    It is because of the fundamental difference between light and sound waves.
    In sound, it is the medium doing the moving.
    In light - it is the electric and magnetic fields.

    There are two basic ways of looking at it. Put real simple:
    1. the speed of light is a kind-of "material" property that belongs to space. It is different in different places depending on the density of electric charges and magnetic dipoles. The denser the material, the more electric charges etc. are in that volume, so the slower the speed of light is there.
    2. light is made of photons that interact with electrons in the material by being absorbed and re-emitted. The effect is that individual photons can be thought of as bouncing around from atom to atom. Light slows in a material like balls are slow getting through the barriers in a pinball machine. The classical "light-ray" path through some material is actually the average of the many possible paths the photons can take.

    ... all on a simple-ish level and, so, incomplete.
    If you wanted more detail I'll need to know more about your education level.
    Last edited: Mar 5, 2014
  4. Mar 5, 2014 #3
    Could someone please confirm that statement? I thought the wave length increases when sound waves enters a denser medium.
  5. Mar 5, 2014 #4

    Simon Bridge

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    Did you have a look at the link at the bottom of post #2?

    The energy in the wave determines the frequency and energy is a conserved quantity.
    In general, though, all waves lose energy as they travel - as their energy dissipates as heat or something.
    The actual transition from one medium to another may involve a refection too - so not all the incoming energy gets transmitted to the next medium.
  6. Mar 5, 2014 #5

    I now see it is energy and frequency and not wavelength and frequency. That statement is correct after all.
  7. Mar 5, 2014 #6


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    Sound waves are carried by momentum transfer; the pressure waves are regions of greater and lesser concentrations of momentum. Try this "model" while thinking of the difference between a gas and a solid.

    For light, the frequency drives the process; so if you were to measure the frequency of light in different media you would find that it is unchanged. Since the effective speed changes, you end up with a change in wavelength: speed = frequency x wavelength. From the photon picture, energy = Planck constant x frequency - thus the energy of each photon is unchanged when it moves from one media to another.

    What accounts for the "slowing" of light? You will find many incorrect explanations, including the purported "FAQ" for the physics forum. The short version goes:

    (1) for a transparent media we know that the light can carry an image - hence the process must be _coherent_.
    (2) for an image to be carried through the media we know that the light travels in the _forward_ direction.
    (3) we know that photon absorption by atoms/molecules requires an available energy level - but this results in resonant absorption, hence we know that image transmission does _not_ rely on absorption
    (4) however, another mechanism is available: photon _scattering_. This process does not change the energy of the photon, though it does change the direction of travel.

    So the answer is _FORWARD COHERENT SCATTERING_. But this is only half of the story. We have not accounted yet for (a) why the speed is slower, and (b) why there is an image at all.

    These require an analysis of the scattering process. Thankfully Christian Huyghens carried this out for us in the 1600s! See http://en.wikipedia.org/wiki/Huygens–Fresnel_principle

    Huyghens principle is a geometric analysis of wave behavior. In the case of photons it is constructive/destructive interference along the phase front produced by the forward coherent scattering.

    Richard Feynman provides a description of this in his famous lectures on "QED: the strange theory of light and matter".
  8. Mar 7, 2014 #7

    Simon Bridge

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    Videos of the lecture series, as given at the University of Auckland NZ:
    ... surprisingly accessible BTW.

    Note: I really need a non-blog, non-wikipedia, source at the same level as the link in post #2.
  9. Mar 7, 2014 #8


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    I didn't like the "explanation" given by the link in #2.

    Here is something at the same level, but better:

    Or this one, which is a bit more technical (introductory physics course in optics):

    This is a nice presentation:

    And for completeness, here is Griffith's explanation based on Maxwell's equations:
    "Why the speed of light is reduced in a transparent medium"

    Full text:

    Feynman's explanation can be read in "QED: The Strange Theory of Light and Matter".
    It is easier to study from the written text.
  10. Mar 8, 2014 #9

    Simon Bridge

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