Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Can sound affect light?

  1. Jan 19, 2009 #1
    I just saw this question on Yahoo answers, and I said no. But another contributor said yes. Then I looked into it, and I'm still not sure if light will be refracted moving through a point of high pressure into a point of low pressure (in air)?. Would the same apply to a point of high to low temperature in air?. I looked on Wikipedia, but it only gave example of light moving through air and entering a solid, or a liquid. If this question is really simple, sorry but I never did well in science :-). I love physics though, I just discovered it too late!. Thanks

    Also, does light use air as a medium?. I'd say no, which would mean the answer to the question above is no also surely?. If light can be affected by Magnetism/Gravity, then so should Sound also???. Any answers would be much appreciated, I am in awe and eternally jealous of those who can contribute constructively and intelligently on a site like this :-)
  2. jcsd
  3. Jan 19, 2009 #2
    I am going to say no because light and sound behaves differently within a medium (as your wiki tells you)

    Edit: ._. wrong again.
    Last edited: Jan 19, 2009
  4. Jan 19, 2009 #3
    Since there's a density difference in both cases it should refract the light.

    You can even see it with the naked eye on big explosions (the shock wave), temperature differences are easier to see like the apparent water on hot roads (mirage).
  5. Jan 19, 2009 #4


    User Avatar
    Science Advisor

  6. Jan 19, 2009 #5


    User Avatar
    Science Advisor

    As far as sound affecting light, I'm not sure. Temperature of air, however, does affect light. Pockets of warm air will behave as a different medium and diffract light a bit when it enters and leaves. This is the reason the air appears to shimmer above hot pavement. I'm not entirely sure if pressure will cause a similar affect, but it seems entirely possible.

    As far as your 2nd paragraph questions are concerned: No light does not use air as a medium (it doesn't need it). Light can be affected by gravity, but is unaffected by electromagnetism. Sound is simply a pressure wave in a given medium, say air. As the individual particles of air are subject to the affects of gravity/em I suppose one could say the pressure wave itself is.

    I wonder if anyone could shed some light on this: Imagine there is a room on, say, Jupiter filled with earth atmosphere. If you tried to use a speaker, would there be any difference in what you hear? (Likely not discernible, but in principle).
  7. Jan 19, 2009 #6
    I may be wrong, but I thought shock waves comes from the fact that the object travels faster than its sourrounding medium. I thought they are different from sound (or at least not classify as simply sound), so it doesn't necc. conclude that sound affects light.

    And can you eleborate on the concept of density difference? density of?
  8. Jan 19, 2009 #7

    Density of the medium light is traveling through, every change in density will cause light to refract. The bigger the change, the larger the refraction. Since sound is a pressure wave there's also a difference in density (higher pressure = higher density)
  9. Jan 19, 2009 #8
    I'd have to guess no, not sure if gravity has any effect on sound, but if it does it's probably a very small effect. If it would then there should be a difference in sound inside a space shuttle or free falling aircraft.

    However if you'd put earths atmosphere on jupiter the air pressure would be higher since it has about 2.54x the gravity. So sound would travel faster there than it would here, making you have a lower sounding voice.
  10. Jan 19, 2009 #9


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    The answer is yes. For those of you who missed it, see atyy's wiki article link:

    Here is a simple explanation:
    Sound wave → changes in pressure → changes in refractive index → refraction of light is affected

    The effect is pretty weak in air. But in a solid crystal (so called acousto-optic modulators), it's pretty routine in optics labs to change the direction and frequency of laser beams using sound waves in the crystal.
  11. Jan 19, 2009 #10
    If you had a drum head that had a mirror like finish, changes in sound pressure could modulate light
  12. Jan 20, 2009 #11
    Thanks for the replies everyone. I'm still confused about this one. I've been doing a little more research, and I've found that (correct me if I'm wrong), light uses Photons to propagate through an un-known medium and Photons have no mass, so how does their travel through a section of low to high density in the atmosphere affect their path if they have no mass?. How does mass (and a change in mass density) affect something which has no mass?. I'm working on the assumption we could say our atmosphere has mass, as opposed to a vacuum. So, heres my crazy theory (keep in mind I'm still new to this), If the strength of gravity is proportional to the mass of an object, and light is affected by gravity, and light is drawn to large objects (dense mass), then could the diffraction be caused by the 'compression' phase of a sound wave, 'drawing' more gravity from within itself, because it has higher mass density than the 'rarefaction' phase of the sound wave?. The higher gravity of the 'compression' phase drawing the light to it?. I'm working on the assumption this would occur on the
    'edge' of the soundwave, I suppose boundaries would have to assumed for this to work??. Am I on the right track?. Basically I still don't see how two disparate mediums could affect each other (Sound using mass, light using no mass)
  13. Jan 20, 2009 #12


    User Avatar
    Science Advisor

    Gravity and light interact, but the effect is extremely tiny, so that most effects can be explained without invoking gravity. The main effects are explained by thinking of light as an oscillating electromagnetic field, which interacts with electrically charged particles in matter.
  14. Jan 21, 2009 #13

    Actually you're way off ;) Light traveling through a medium is slowed down, the amount it slows down depends on the density of the medium. The reason for this is that a photon just like anything else can't move through matter. What happen is the photon keeps hitting particles and the particles keep re emitting the photon (if it's a transparent medium of course, if it's not the photon is absorbed). Because it takes time to absorb and emit the photon the light beam is slowed down. Thus the more particles there are in a region of space the more times the photon will hit something, the slower it will go and hence the larger the refraction will be.

    Anyhow, long story, probably better explained here:
    Last edited: Jan 21, 2009
  15. Jan 21, 2009 #14
    Actually it's not absorbed and re-emitted, common fallacy. Read the FAQ on this very forum for details.


  16. Jan 21, 2009 #15
    According to this part in the same post it actually is, unless I misinterpret it.
  17. Jan 21, 2009 #16


    User Avatar
    Science Advisor

    This part is rather clear. The lattice does not absorb the photon.

    For a completely correct solution, which also satisfies nitpickers, one should use polaritons (quasiparticles describing two other coupled particles) as the representation of the em field inside some materials. That solves some of the ambiguities. However this is getting a bit off topic, so I won't go into detail here.
  18. Jan 22, 2009 #17
    Yeah the key here is absorbed and re-emitted. I wouldn't worry it's I suspect it's in the FAQ for very good reasons, just about everybody learns the wrong model in class all over the world. I just happened to read the FAQ before posting here. Just like at some time everyone Learns the Planck or Bohr models of the atom with their planets spinning around the nucleus, which are equally not quite right.
  19. Jan 30, 2009 #18
    I am no expert but "Physics Curious" I came here looking for the reverse of this question. Does light have any effect on sound? More to the point, can light either speed up sound and/or -- forgive me, I don't know how to ask this properly-- provide a smoother surface for sound waves to more easily travel-- as if sound waves would travel better by hitching a ride on light? Does light provide a more slippery surface for sound? (slippery surface is the best analogy I can think of). Does anyone have an idea about this? Or how to present this question? Should I start a different thread? Thanks for your time!
  20. Jan 30, 2009 #19
    If the light is losing energy to it's surroundings as it would in an atmosphere then yes, sound travels differently according to the density of a medium which is effected by temperature, most of which comes from the Sun, and much of which is light. So yes in theory it would be affected indirectly. I don't think it would be effected in any direct way though.

    Don't forget in space no one can hear you scream. Sound needs a medium to propagate and is inherently related to that medium.
  21. Jan 30, 2009 #20
    I appreciate the answer, The Dagda, this of course leads to more questions and I'll follow the lead where it leads, I suppose. :smile: Thank you for your time!
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?