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Can any (Physicist) explain this laser-weirdness that I'm seeing? Answered

I've got some cheap-o red-lasers.
Up close on surfaces I see what looks like old-fashioned monochrome TV static in miniature* (and red).
Other people see it too, I think it's some kind of "Quantum-noise" but would like other opinions.

L

*I don't like looking at it - it reminds me of this.

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kelseymh

Best Answer 6 years ago

Hi, Lemonie. Everyone else has given you the correct answer. It's a speckle pattern, caused by interference of the beam with itself over the path from the laser to the surface to your eye.

The observable interference itself occurs when the beam gets to your eye, not "at the reflector." Because your eye is constantly saccading (jittering), the interference pattern on your retina varies, and you see the dynamic "static" you described.

You will see a speckle pattern for both rough and (optically) smooth surfaces, because the beam itself has a finite extent. Consequently, there are path-length differences from a ray on one side of the beam interfering with a ray from the other side, for example.

Naively, you might expect all those phase differences to lead to destructive interference, but of course it's more complicated than that. Energy conservation implies that you can't just have the power "go away," and in fact averaged over the beam spot, the constructive interference from nearby rays dominates over the random destructive interfence; a rather similar effect to Feyman's path-integral formulation of QM, and the path-integral derivation of Snell's law.

The key to recognizing a true speckle pattern (the fact that it is beam-scale self interference) is that you can't magnify it. You'll see the same apparent-size "static" spots whether you shine the beam on the desk right in front of you, the wall behind you, or a projector screen 30 meters away.

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lemoniekelseymh

Answer 6 years ago

That's the sort of explanation, which I understand, that I expected.
Thanks

L

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lemonieCameronSS

Answer 6 years ago

Yes, I think that I get it, but the static would be arising from my hand moving then?


L

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CameronSSlemonie

Answer 6 years ago

Hand movement, vibration in the surface, turbulence in the air to distort the beam--as far as I understand it, just about any change in the optical path will change the appearance. Random speckles constantly changing to other random speckles = poltergeists.

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steveastrouk

6 years ago

Yep, its "speckle" as Iceng and Cam say.
Its an interference effect, caused by small variations in pathlengths from the bumps on the surface.

You can sometimes see it in full sunlight on a newspaper, but its much easier to see in monochromatic light.

Steve

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lemonieJamesRPatrick

Answer 6 years ago

No, I don't have a camera that would do it. But it looks like this in red.

L

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iceng

6 years ago

The speckle also occurs from my o.5mw He laser which I understand results from scattering of the monochromatic beam by a surface that is rough on a microscopic (wavelength) scale.  .  .  .  .  .   A

specklinglaser1.png