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Q & A: Laser induced dots?

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Most recent answer: 04/26/2008
Q:
Hi! - I have a science background but for many years now, have not been directly connected with my earlier activities. I have noticed that when I hold a small laser pointer close to a surface - my finger-tip, a table, or any other object - there is a distinct and very active halo with myriads of micoscopic, black, closely-packed dots in intense and erratic movement. The similarity to the well-known Brownian movement is striking and it occurred to me that, indeed, this is what is happening. It has been suggested to me from another source that this is merely dust-motes floating around like the dust particles in a sunbeam or moonbeam (or any other ray of light) but in my opinion the intensity and infinite number of particles preclude such a simple explanation. Any comments? Yours, Selwyn Rose. p.s. How will I know if my question prompts a reply? s.
- Selwyn Rose (age 72)
Israel
A:
The only thing I can think of is that you may be experiencing the phenomenon of 'floaters' which are bits of flotsam and jetsam in the eyeball itself.  I've seen them myself.  The laser beam itself shouldn't have any effect per se.   
Take a look at  http://www.allaboutvision.com/conditions/spotsfloats.htm  and see if this is relevant to your experience.

LeeH

I would guess another phenomenon, although it would help to have a little better picture of what you're seeing. Laser reflections often show an oddly undulating blotchy dark-light pattern. The pattern moves in a strange way as you move your head. This results from coherent interference between the laser light scattered off different things. The light reaching some part of our eye can consist of light following slightly different paths, with the wave-fronts either accidentally adding up or accidentally canceling. An ordinary light source would show a similar effect, but its pattern is changing so rapidly that you just see the blurred-out average, with all the blotches removed. Mike W.

(published on 04/26/2008)

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