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If a laser beam is looked at from the side versus a dark background, a sparkling effect can be seen caused by dust particles in the air hit by the beam.

Is there any simple model or coarse estimations how often that would happen, how bright the particles flash and how long a single flash will be?

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What do you need it for? Is it for the problem itself or you need to solve another problem? –  Yrogirg Aug 15 '12 at 8:21
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Your question encompasses many fields: Light scatering by small particles has two main branches: Rayleight scatering and de Mie scatering. How often light scatering happens is studied in radiative models and necessitates intregro-differential equations.

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As far as I recall Mie and Rayleigh cases are for cases when particles are comparable or smaller than the electromagnetic wavelength. I think OP described ordinary reflection on quite large particles, which should better explain sparkling. Do you think there would be much sparkling for the Rayleight scattering? –  Yrogirg Aug 15 '12 at 8:20
    
Yes, there are definitely 'large' particles involved, like dust and mist drops, which may be invisible to the bare eye, but are visible as single particles if illuminated brightly. So I think my question is not so much about the scattering details but more about the statistics of particles in the air, their size and their reflection ability (maybe just their 'color'?). –  dronus Aug 16 '12 at 14:01
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