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Hearing light

  1. May 14, 2008 #1
    Something I've been thinking about, and I just have no idea in which forum to post it. If the sensation of sound is due to the collisions of molecules against your eardrum, and they impart momentum transduced across your eardrum into phonons which occur in 'solids' as modes of vibration treated as bosonic particles that also have momentum, then could electromagnetic radiation of sufficient intensity and hi-frequency impart enough momentum to trigger sensation of sound? Is it really all due to momentum? My own smart-ass reply is: "Yes, it's called sight", but of course I mean literally sound. And what would light sound like?

  2. jcsd
  3. May 14, 2008 #2


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    Sound is a longditudinal wave i.e. compressions and decompressions in the direction of motion. Sound is not caused by the collision of molecules against your eardrum. Your eardrum can interpret certain frequencies of the londitudial waves of compressions; caused by some mechanical movement.

    So in this sence, as light is a transverse wave (oscillations are perpedicular to direction of motion) and the oscillations are electromagnetic, your ear could not interpret EM waves as sound.

    However, light is dicretely packaged as photons which carry momentum and as such a momentum transfer bewteen, say, a solid surface and light can and does occur(I know little about this)
    . But again, any sound which arises from this would be due to the mechanical movement of the target medium producing compressions in the air etc etc, so the sound is not a direct result of the light.

    Photomechanical effects are used also in medical application to break up kidney stones and other things by using short pulsed lasers to rapidly vapourise tissue causing shockwaves (at certain pulse durations).

    But to answer your question, light can only give rise to mechanical effect which can cause sound (vibrations); and cannot dirrectly induce a sound, only secondarily.
    Last edited: May 14, 2008
  4. May 14, 2008 #3

    I know that's the classical description of a sound wave in air. This is what all the engineers and physicists say. So would that then mean "sound" is purely a macroscopic phenomenon (not the perception of sound, but the 'essence' of sound)? An example of such a macroscopic-only phenomenon is entropy. If you view a molecular collision it appears reversible in time, and only when viewed on a 'large' scale is entropy apparent. So, would a single molecule or couple molecules bouncing around a box be entirely soundless because there is no way to model rarefaction and compression as with a population of molecules? I ask this because the longitudinal 'waves' produced by atmospheric pressure differentials ultimately do cause molecules to pelt your eardrum which triggers the sensation of noise (this is why we hear 'wind'). My suspicion was that even when considering a molecule or 2, the impulse of collision would be sufficient to produce phonons (if momentum was high enough). Hence, momentum would be what to consider and that opens the question to light which though massless, has momentum.
    Last edited: May 14, 2008
  5. May 14, 2008 #4
    Light does exert pressure, and by modulating the intensity of the light you could in effect cause a transducer to think that sound was impinging on it. I have a feeling that the intensities needed would cook your eardrum, but maybe this has other applications.
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