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The diffrence between refraction of sound and light

  1. Nov 21, 2012 #1
    I was thinking about refraction of waves and I´m a little unsure. I wanted to run my understanding through you guys to make sure I´m right.

    So

    The general rule is when a wave goes from a slower medium to a faster medium the wave should turn sligtly away from the faster medium when it enters it. And we can turn it around and say when a wave goes from a faster medium to a slower medium the wave should turn slightly towards the medium when it enters it.

    Something like this:
    http://hyperphysics.phy-astr.gsu.edu/hbase/geoopt/refr.html

    So when a light wave goes from air to water the light should bend towards the water because it going from faster to a slower medium. But it should be the opposite with sound, when sound goes from air to water then the wave is going from a slower medium to a faster medium so it should bend away from the water when it enters it.

    Am I right about this? And if I am right than this picture must be wrong wich confused me alot.
    http://learn.uci.edu/oo/getOCWPage.php?course=OC0811004&lesson=005&topic=003&page=14
     
  2. jcsd
  3. Nov 21, 2012 #2

    mfb

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    Why? Sound is just like light in that respect.

    It is different for matter waves, but that is quantum mechanics.
     
  4. Nov 21, 2012 #3
    Sound refracts the opposite way as opposed to light because when sound is entering water from air it is going from a slower to faster medium, but when light is entering water from air it is going from faster to a slower medium and should refract the opposite way. Right?

    That is why I said sound should be the opposite because i was refering to sound refracting in the opposite direction as opposed to light when entering water.
     
  5. Nov 21, 2012 #4
    The image you show is indeed not realistic for sound waves.
    When sound enters water from air the angle between the normal and the beam increases.

    Here is a nice site where you can experiment (virtualy):
    http://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/Physics/refractionsnells.htm

    I think the problem may originate in the author of the site reproducing an image from Dr Russell's site (they give a link to it, in blue) without realizing that that specific image illustrates refraction of light.
     
    Last edited: Nov 21, 2012
  6. Nov 21, 2012 #5

    mfb

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    Ah ok, your remark was specific to water/air and not related to the general concept. Well, that is right.
     
  7. Nov 21, 2012 #6
    Thanks for the reply. I think you are right, must have been an oversight on there part, but it confused the hell out of me.
     
  8. Nov 22, 2012 #7
    Hi Sjonson, welcome to physics forums! :smile:

    It is the same for all waves. And instead of learning it with words, it is much more useful to understand it with the Huygens construction:

    https://en.wikipedia.org/wiki/Huygens'_principle
     
  9. Nov 22, 2012 #8

    sophiecentaur

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    The angles involved depend on the Wavelength - which, of course, relates to speed. In water, the wavelength of sound is about fifteen times the speed in air so it would be quite difficult to do a scale diagram what would make immediate sense. Of course, 'that diagram' is the wrong way round and, imo, a rather sloppy bit of presentation - if it is trying to be helpful.

    There is another very important issue with the passage of sound from air into water. When waves pass from one medium to another, there is always some reflection of energy (a reflected wave). The greater the difference in refractive index, the more is reflected. In the case of Air into Water - or the other way, the mis-match is huge and most of the sound energy is reflected. That's one reason why it all goes quiet when we stick out heads under water.
     
  10. Nov 22, 2012 #9
    This did not come out very well.
    The speed in water is about 4 times the speed in air (a little more than 4).
    So the wavelength in water is about 4-5 times larger than the wavelength in air (for the same frequency, of course).
     
  11. Nov 22, 2012 #10

    sophiecentaur

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    Whoops - I was quoting the water speed in ft/s Durrrr. Why can we still encounter those pesky Imperial Units on the Webb?

    The other bit was correct though!
     
    Last edited: Nov 22, 2012
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