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Sound and light

  1. Jul 5, 2004 #1
    I am researching sound at Concordia University, Quebec. Recently, my mind was wondering and I thought about sound being produced by the air molecules bumping into each other. I began to wonder what else could do the same thing.
    Does light have contact with the air molecules in this same way?
    Or does a "laser"?
    There is this tool guitarists use called a an "ebow" which is a sort of "lazer" that is placed over the string and ends up vibrating the string. This is the sort of thing I would like to understand better?

  2. jcsd
  3. Jul 5, 2004 #2


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    Sound is not produced by air molecules bumping into each other; sound is transmitted by air molecules bumping into each other.

    A laser ordinarily cannot be used to push air molecules around to get sound. The best you can do with a laser is randomly scatter electrons.

    And the EBow is not, in any respect, a laser. It's a purely electronic device that uses an electromagnet to wiggle the guitar string.

    - Warren
  4. Jul 6, 2004 #3
    Alright, my mistake in over generalizing the way we percieve sound. But is there anything else that could act in the same means as the way sound is transmitted?
  5. Jul 6, 2004 #4


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    Pretty much anything that works the way sound does (a longitudinal pressure wave) is also a sound wave. Some types of earthquakes, for example.
  6. Jul 12, 2004 #5
    Hello everyone. Intrestting question. From what i know, sound isnt the bumping of air molecules, because air molecules are randomly colliding with each other as we speak, according to the kinetic molecular theory. Sound is areas of high and low pressure of air molecules, which is basicly the wave. About the light...I dont think it can cause sound, because of basicly what i said in the biginning. Since light can simply give more kinetic energy to the molecules; making them have a faster rms, and therefore colliding more often. So basicly you're affecting the bumping of the molecules, not really affecting them in a wave to transmit sound. Anyways, that's how i see it. :D
  7. Jul 21, 2004 #6
    Sound transmission is the bumping of air molecules in a coordinated manner, which translates in alternating regions of high and low air pressure. Such alternating regions are called waves. Such waves can happen in any kind of solid : gaz (air), liquid (water), or solid (metal, wood, eardrum).

    The ebow makes magnetic waves, which makes metal waves (string), which makes air-pressure waves, which makes eardrum waves, which makes waves in the internal ear liquid, which makes cells vibrate and tell our brain something is going on (noise). Any time any solid moves, there is a wave, that can potentially reach our ear and thus make a noise, be it whatever solid you can imagine (including earth, in the case of earthquakes).

    As for light and lasers, here is how it can produce sound waves:

    - A high-energy laser pulse hitting metal can cause is to quickly heat up and rapidly expand, causing a high pressure air zone that will wave its way to your ear, making a loud "CLING" noise. This is cool (but annoying).

    - An sufficiently-high power laser pulse can ionize air molecules at the beam's focal point. As for lightning, this causes rapid expansion and makes a loud "THWACK" noise (for one pulse, lightning is a series of such "THWACK's", corresponding to a large distance rather than a focal point).

    - Light has momentum. It is thus possible (it's been done) to use laser light to lift a tiny (100 microns) sphere of glass in mid air. I suppose you could push it against a drum and make a small noise...
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