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Old 01-15-2013, 12:39 AM #17
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Default Re: Why does laser light look like little particles moving about?

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Originally Posted by tommmtom View Post
Just curious, not sure if it's just me, why does laser light appear to be little particles on the surface you point it at?
Reading is the key



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Old 01-15-2013, 01:04 AM #18
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Default Re: Why does laser light look like little particles moving about?

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Reading is the key
Silly me. Overeager with my photo there.
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Old 01-18-2013, 07:12 PM #19
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Default Re: Why does laser light look like little particles moving about?

The speckle pattern is easier to see and photograph when viewing the secondary reflection.



If you take the output of a HeNe laser, bounce it off surface A, and onto surface B. What you see on surface B looks much like the photo. As long as the surfaces and the light source are stationary, the speckles will not move or change. However, you will see secondary speckles that do move. Speckles within the speckles.

The photo (above) was apparently taken by bouncing the beam off the back of a cellphone (plastic surface) and on to a wall.

It is funny this comes up, because I have memory of a phenomenon that I want to demonstrate to my kids, but have failed. When viewing the spot on a wall from a red HeNe laser, if I slowly rotate my head to the right while keeping my gaze focused on the spot, I see the speckles moving to the left. No surprise, you might say. Light enters my eye and gets inverted by the lens. Thus, on the back of my eye, things are inverted across my retina.

But here is the kicker. I recall looking at a different wavelength of light from a different laser, and noting the speckles move up and down, as I rotate my head to the left and right. But, for the life of me, I've been unable to recreate this phenomenon for my children. I originally saw this 20 years ago, so maybe my memory is failing.

I've only worked with a few kinds of gas lasers, in the past. If I am remembering correctly, this could only have occurred with a HeCd, Kr or an Ar laser. My money is on the Argon laser. I'm looking for one, btw.
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Old 01-18-2013, 09:32 PM #20
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Default Re: Why does laser light look like little particles moving about?

The direction of movement of the speckle pattern depends on your movement (if any) and whether/ how far your eyes are focused in front of/behind the surface. I have found that the speckle pattern moves one way when my eyes are focused closer to me from the surface while I am moving, and moves in the opposite direction if my gaze is focused beyond it while I am moving (unless I reverse direction simultaneously). If I focus at the surface the granular appearance changes but not with a directional shift, a random kind of "boiling" motion.

Fun to play with this phenomenon!

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