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Featured I Do light and sound waves roll up and break like ocean waves?

  1. Aug 26, 2017 #1
    When sea waves approach the shore they roll up and break due to different velocities of water layers formed due to the gradual change in water depth. The highest wave peaks move faster than all other layers and thus falls down. All other layers fall the same way but in a delay. this ends up with wave rolling up when approaching the shore.

    Does light and sound wave have a similar phenomenon? and then how it looks like or how it sounds?
     
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  3. Aug 26, 2017 #2
    Hi there. This is my first attempt at a reply. I hope others can correct me if I am wrong. As sounds waves are compressions and rarefactions (compressions and streaching) in air. Wouldn'the the collapse you talk about occur when we hear a sonic boom of an aircraft breaking the sound barrier ?. As for light this would be a different case as its speed is a fundamental constant of nature (no eather to support a collapse).
     
  4. Aug 26, 2017 #3

    tech99

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    I sometimes think that the breaking wave is a conversion from a transverse wave, going up and down, into a longitudinal wave, going forward and back.
    Such transitions occur with EM waves with a receiving antenna wire, where an incoming transverse EM wave is partly converted to a longitudinal wave on the wire. In sound, a transverse wave on a string is converted to a longitudinal wave in the air.
    There are a number of situations with EM waves, light etc, where the medium no longer supports a transverse wave, and we get an evanescent wave, so called. This includes when the wave enters a metal pipe that is "too small" and at a surface where there is total internal reflection.
     
  5. Aug 26, 2017 #4

    I like Serena

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    With light we have for instance Cherenkov radiation, which happens when particles enter a medium where their speed exceeds the speed of light in that medium.
    (Contrary to popular belief, the speed of light is not an absolute limit - it's the theoretical speed of light in vacuum that is.)
    Although, to be fair, that's not about actual light going through mediums with decreasing light speeds, but about particles with speeds approaching the speed of light.
     
    Last edited: Aug 26, 2017
  6. Aug 26, 2017 #5
    Something to think about is the fact that water is not very compressible so that is why it rolls up and crashes when it reaches a shoreline, sound waves are different in the fact that they do compress and stretch the matter they travel through...
     
  7. Aug 27, 2017 #6
    This is how it looks with light:
    p5p7m34opi2d7gixao05.gif


    The "photonic Mach boom" is produced when a light wave passes through two different media, where one has a slower velocity of propagation than the other.
     
  8. Aug 27, 2017 #7
    Fantastic, so you do get a light boom.
     
  9. Aug 27, 2017 #8

    sophiecentaur

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    Ocean Waves are different from sound and EM waves - or even waves on strings. They are Surface Waves, which involve transverse AND longitudinal displacement (particles follow an elliptical curve as the wave passes*) and you can get 'overtaking' of the water at the peaks when it falls forward, in front of the lower parts of the wave. This overtaking of the medium can't happen with transverse or longitudinal waves so there is a fundamental difference. In the case of a shock wave, the particles of the medium are pushed forward at a speed greater than the natural wave speed.
    * I remember scuba diving over a bed of very long kelp, under some significant swell. There was a finger of rock sticking up above the kelp and I could see the top (from my reference frame) performing a massive vertical circle in front of me. Bizarre. The illusion was magnified by the fact that the kelp was moving with me all the time -just the rock was bolted to the sea bed. It was all very peaceful with no fuss - till I was getting back into the dive boat.
     
  10. Aug 28, 2017 #9
    Dear all replayers
    1. The different wave velocities in a medium are not sufficient to have wave roll up. You need a force field (like gravitation) to have an equivalent to a water layer falling due to gravity.
    2. Remeber the water bath with slits that showing the diffraction and interference the geometrical considerations in bought light and water waves going through slits are the same. This is one of the equivalents that makes us think the light has wave aspect.
     
  11. Aug 28, 2017 #10

    CWatters

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    So could it happen near a black hole?
     
  12. Aug 28, 2017 #11

    tech99

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    (1) So far as I know we do not have any wave breaking action with EM waves if we use a magnet or a fixed electric field. (I realise we can have polarization effects if the medium is a dielectric). However, I think that the action of receiving and transmitting antennas are equivalent to wave breaking.
     
  13. Aug 28, 2017 #12

    sophiecentaur

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    Isn't it more basic than that? To get a breaking phenomenon, you need parts of the medium to overtake other parts. How is that going to happen within the bulk of any medium unless there is a path 'round the outside" (over the top, actually)? It would have to involve Turbulence which is hard to envisage along a uniform wave front. Turbulence happens with supersonic aircraft but I can't see that is the same as a water wave breaking.
     
  14. Aug 28, 2017 #13
    As I see it, the waves breaking on a beach happens as the base of the wave meets resistance in the form of the sloping seabottom and beach and the top of the wave, still carrying it's same forward momentum is going to go faster than the base of the wave, thus the (as we see it) curl and break of the wave, most of this energy is changed into Sound, both in air and through the ground. Both are physical waves.

    The same basic mechanism is happening with both sonic booms and Cherenkov radiation, where the wave that is produced is moving faster than the media it is in can accept thus the crash and boom of the supersonics as the object creating the wave is moving faster than the air can carry that wave and so it is changed radically in volume as the speed energy is dissipated/transformed into something that the air mass can carry, thus no energy loss. Fast becomes Loud.

    The same happens with the blue glow of Cherenkov radiation where light ( akin to high energy gamma rays) or particles (akin to cosmic rays but from radioactive sources) are trying to go through a medium with more energy and speed than is allowable and the energy from the slowdown because of this resistance is transformed into a different range of light which we see as a blue glow.

    Admittedly, they are all in different regimes, but the basic cause is the same and that is the energy the initial wave carries being more than the wave-form can carry in that medium and that extra energy is transformed into a different but similar form of energy which the media Can support.
     
    Last edited: Aug 28, 2017
  15. Aug 28, 2017 #14

    sophiecentaur

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    I can't see how there is any re-ordering of the medium when there is no 'extra dimension' for the material to move through (i.e. the air above the water surface. After a water wave has passed, the water at the surface has actually changed places with water in front and behind. In an ordinary sound wave, the medium returns to the same distribution as it was before the wave went through.
    To my mind this is highly significant and it is a different mechanism for resolving the problem of different wave speeds from an ordinary longitudinal wave.
     
  16. Aug 28, 2017 #15
    "So could it happen near a black hole?"
    I have no idea.
    I think in most cases scientists observed a phenomenon and then did the analogy. I do not recall any case of a searching phenomenon to meet analog.
    In analogy to mathematics. It is much easier to obtain a derivative than to integrate.:smile: The analog is a "derivation" of a phenomenon:wink:
     
  17. Aug 30, 2017 #16
    For sound, it is shockwaves.

    The basis of the phenomenon is that wave velocity depends on the amplitude/phase, such that for sufficiently strong waves the wave crests move faster and overtake the bottoms.

    Note that the non-sinusoidal, broken waves can be approached by Fourier transform as combinations of waves with higher harmonics, so frequency is multipled.

    Are there any conditions where light gets broken - as in, the waveform is changed by the crests overtaking other phases of the wave? Strong light does feature generation of light harmonics, but does light form shockwaves then?
     
  18. Aug 30, 2017 #17

    sophiecentaur

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    Fourier analysis only applies to single valued functions. After the sea wave has 'broken', there is water 'hanging' over the trough and that corresponds to four[Edit: sorry, Three!!] values of displacement at that position. The analogy doesn't apply to any other form of simple wave.
    PS Has no one else ever seen a breaking wave and observed that the peak moves out over the trough? No surfers????
     
    Last edited: Aug 31, 2017
  19. Aug 30, 2017 #18
    In my opinion we have to decide whether we have to look at sound and light as particles or waves. I believe this will simplify the question a lot.
     
  20. Aug 30, 2017 #19
    From past experience in this forum, the particle/wave aspect only complicates matters and causes confusion!
    Sound waves consist of waves in matter they propagate through, having no "substance" of their own to call a "particle".
    Electromagnetic waves don't have particle behavior until they interact with matter, and they always travel at the same velocity in any given media so the only thing that could possibly "roll up and break" might be the phase velocity, which can exceed the speed of light but I don't know what visual effects that might incur.
     
  21. Aug 31, 2017 #20

    sophiecentaur

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    Sound propagation through a solid can be analysed very successfully in terms of Phonons. (link) It's strange that no one 'wants' phonons to the real; they accept that they are only a convenient model and happily get on with "Sound Waves". Photons should have a similar status because they are only one convenient model of EM propagation and interaction.
     
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