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Infrared Laser spinning a small paper

  1. Apr 16, 2007 #1
    Firstly, this is NOT a homework question although it may sound like it. My question pertains to this (I believe rather cleverly faked) video.

    My question is this: Could an Infrared Laser (there are varying power output lasers of 2W - 20mW) SPIN a paper of say 25 grams as seen in the vid on the tip of a needle/toothpick? I am trying to put forth a plausible scientific mechanism for reproducing this effect.

    Now I don't know if I'm barking up the right tree so-to-speak but Power=torque x angular speed, so I'm hoping somehow that we can show that such a laser could in theory or in practice push the paper and keep it spinning.

    What other scientific mechanisms could you suggest that could reproduce this effect? There is obviously the possibility of something hidden in the table with miniscule holes blowing out air but then the (what I believe) trick gets a little too elaborate ie. remote control + tiny air pump etc.

    Your help is much appreciated.
     
  2. jcsd
  3. Apr 17, 2007 #2
    As you realise, countless mechanisms could be applied to perform the trick.. but at the end of the day, all you have is a video. Since you can't know how much the video may have been "doctored", there's not much point trying to figure out anything further.
     
  4. Apr 17, 2007 #3

    Mentz114

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    The first 'demonstration' of light pressure turned out to be a heating effect. Using an IR laser will also cause uneven heating and an impulse. If they can do it in a hard vacuum with a blue laser I'll believe it.
     
  5. Apr 17, 2007 #4
    Thank you Mentz114. I suspected that it might heat it rather than push it.

    So an IR laser might be out of the question but not to worry I've just found a manufacturer of DC operated micro air pumps that would fit snuggly into the table and could easily be operated by RC. It would only require miniscule holes in the table to direct the air (not much pressure needed either) at the paper and hey presto. :smile:

    Cheers
     
  6. Apr 17, 2007 #5
    I do not think that it is a doctored video.
    It reminds me a hoax that Martin Gardner published in Scientific American some thirty years ago (yes, I'm that old). It consisted to make a similar device turn putting your hand around it.
    And, of course, it turned.
    The reason is that the friction forces are very small and, as the paper is hold by the point of something, torques due to friction are very small. Any torque produced by convection (thermal) air displacements is prone to make the device turn.
    The idea of an IR laser is most plausible. And I agree with Mentz114 that it is uneven heating and convection currents.

    The device is (still) called a "radiometer" and can be bought in tourist souvenir shops. It is, as you said, thermal, but it works at low pressure. It is the shock of air molecules, which is different between the brilliant and blackened faces, that makes the device turn. It cannot work in good vacuum. It needs IR light (found in usual lighting) and not blue or UV.
     
    Last edited: Apr 17, 2007
  7. Apr 17, 2007 #6
    Thanks lpfr. I think I misunderstood Mentz114's response. Are you saying that it would be possible to create the effect seen in the video by directing an IR laser (located off-screen) at the paper? In effect the paper and the air around it is heated quickly and creates convection currents that will cause the paper to spin?
     
  8. Apr 17, 2007 #7
    Yes, Guardian. This is my bet.
     
  9. Apr 17, 2007 #8
    One issue with that theory, most cameras will pick up IR.
     
  10. Apr 17, 2007 #9
    Thanks :wink:

    That's interesting. I notice though that some IR lasers produce beams of wavelength as high as 1300nm which is way above the ~700nm for visible light.
     
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