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[vid] Some cool phenomena with laser beam

c4r0

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It's a laser spot on a wall from 100mW DPSS laser in distance of about 150m. I was putting my hand into the beam standing about 30 meters from the wall. You can see two different phenomena on this video.

First one is that moving patterns (I don't nkow how to name it :p) above my hand. It's caused by air that my hand warmed and humidified. The refraction ratio of that air is a bit different than refraction ratio of surrounding air so the light beam is curved.

Second phenomenon it not clear to me, I marked it with arrows. It looks like some kind of interference patterns. I think that it may be that indeed. When light passes very close to some solid object it becomes a bit curved in that place. So the laser light that passes near my fingers edges is curved and since then it's way is not parallel to the rest of light. In this situation (when two beams of monochromatic light coming from the same source are projected on a screen at slightly different angles) interference stripes appear and this is what's going on here I think. Do you guys have any better explanation for it?
 

Benm

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I think its light that just grazes your fingers, and is reflected there. Many surfaces that appear hardly reflective from a normal angle, can reflect light pretty well at really shallow angles on incidence. These reflections will have an effect on interference patters (speckle), and i think that's what causes these lines.

The reflection effect you can easily demonstrate: Put a piece of plain copy paper on a table, and a laser pointer next to it. Point slightly down so you make a line on the paper, and look at the wall behind it - you'll see a reflected plume-like smear of light, even though the paper was matte to begin with.
 

Switch

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Wow, the "heat wave" effect is pretty cool! How cold was it outside? And do you really need huge distance or (I'm thinking) it would work just as well if you unfocused the laser from short range?
 

c4r0

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It was about 5-10 celsius degrees and the distance is needed. If you want to see such thing in shorter distance you can try passing laser beam above a flame of candle or lighter - the spot will flicker on wall or some other screen in 2-3 meters distance :)

Benm said:
I think its light that just grazes your fingers, and is reflected there. Many surfaces that appear hardly reflective from a normal angle, can reflect light pretty well at really shallow angles on incidence. These reflections will have an effect on interference patters (speckle), and i think that's what causes these lines.

The reflection effect you can easily demonstrate: Put a piece of plain copy paper on a table, and a laser pointer next to it. Point slightly down so you make a line on the paper, and look at the wall behind it - you'll see a reflected plume-like smear of light, even though the paper was matte to begin with.
Yesterday 02:44 PM
You're right, it works and it may be it :)
 

Benm

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I think the thermal effect is so pronouced because it makes the speckle pattern jump around too... Normally you would have to deflect light significantly to notice the distortion (like over an asphalt road in summer), but here changes in path in the order of the wavelength are sufficient to be visible. I'm not entirely sure on this though, seems to warrant further experiments :)
 




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