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Retro-reflective smoke

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Not really smoke, per se. I was messing around with some sulfur, heating it up enough that it had melted and was boiling a bit. The sulfur was not burning, but was evaporating due to heat. The "smoke" therefore consists of solidified sulfur particles, rather than combustion products. I'm not sure if the particles are crystalline or amorphous spheroids, but either way they should be fairly pure sulfur.

As I understand it, both crystalline substances and amorphous spheres can act as retro-reflectors. Crystalline things act as corner reflectors and spherical retroreflectors are what is used in reflective clothing and whatnot. Here is what I noticed:

The vaporizing sulfur did put particles into the air, but not very many. Certainly far less than would be given off by incense. The visibility of outgoing laser beams was much higher compared to using combustion smoke to scatter the light. However, the visibility was only enhanced when the beam was viewed along and near the beam axis. More than 45 degrees of angle resulted in much less enhancement, with no enhancement at all when I viewed the beam perpendicular. Clearly, the particles tended to reflect the light back 180 degrees, thus acting as if they were retroreflective.

The visibility of an oncoming beam (reflected from a mirror) was also enhanced, but it wasn't much brighter than the outgoing beam. I tried this with red, blue, green, and violet lasers. The violet laser was barely affected at all, but it's a fairly low power unit anyway.

Interesting effect though, figured I would mention it.
 

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I wasn't burning the sulfur, just heating it enough that it was vaporizing a bit. It didn't create anything corrosive or toxic. Sulfur itself is relatively benign.
 

DrSid

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What are the scattering intensities when looking toward laser ? Reyleigh scattering has quite strong directivity too .. but it is symmetric .. it scatters both toward source and along the beam.
But it usually works on molecules alone, nothing bigger.
 
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If you could document this visually, I'd be very interested in looking into this as well. Looks very cool!

An interesting effect indeed judging by your description :)

Did you try with different wavelengths to see how they each reacted? Perhaps there is a gain curve showing the various reaction capabilities of certain wavelengths?
 
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I tried it with my 638, 532, 450, and 405 nanometer lasers. Far as I could tell, the retroreflective effect was essentially the same for all of them, and the relative visibilities were what we would normally expect given the wavelengths involved. The 405nm was the least visible, but was still enhanced a bit.

I did examine the lasers with the beams coming towards me. The effect was similar to regular smoke, but a bit more "sparkly", sort of like pointing the laser into falling snow, but with much tinier particles. The blue and violet lasers exhibited their usual Rayleigh scattering, while the red was mostly scattered by the airborne particles. The green was brightest of all, as green is wont to be.

In all cases though, the reflective effect diminished greatly, almost to the point of not being there, when the line of sight was perpendicular to the beam. There was still some scattering, but not much. The red laser's visibility from a perpendicular viewpoint was barely brighter than it would have been in clear air. Going from viewing on-axis to perpendicular and back made the beam appear to fade in and out. Very interesting.

I didn't expect this effect to occur at all, I was just fooling around making the sulfur evaporate because I was bored at the time. Viewing from the side and noticing the beam visibility diminish to nearly clear-air brightness was purely accidental, and very surprising.
 




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