How works a beam?

Maybe this isn't the question to start with on this forum, but i have a 'fight' about this, and i know for sure i'm right .

You can see the beam of a laser (from +/- 5mw) in the dark, without smoke or something, right (please confirm)?
Now, how does that work? Are there little dust 'things' in the air or something? My friend claims you need something to reflect to, with which i agree, so, i don't know also how it works.

Thanks in advance.

Yep, it's just air pollutants. There's really nothing else to it. IE, if you shine a laser in a vacuum, you won't see a beam at all.

Okay, thanks, whee, i have won!
Well, i'm going to buy my first laser (a 5mw) soon, so, i stay here .

Thanks for the answer!

You will have to buy a 5mW green laser if you want to see the beam, you won't see the beam coming from a 5mW red laser because red is not as sensitive to the human eye as green.

Yes, that's something i found out searching around. So, that's going to be my first laser.
And of course a 'real' 5mw, not an 'ebay' 5mw.

not gonna happen with a 5mw....you can see it but not too clearly.....just get a dx 30

Chad said:

Yep, it's just air pollutants. There's really nothing else to it. IE, if you shine a laser in a vacuum, you won't see a beam at all.

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hey, I just tried this - your vacuum must be cleaner than mine. Or was I supposed to empty the bag first?

danq said:

[quote author=Chad link=1210116638/0#1 date=1210116953]Yep, it's just air pollutants. There's really nothing else to it. IE, if you shine a laser in a vacuum, you won't see a beam at all.

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hey, I just tried this - your vacuum must be cleaner than mine. Or was I supposed to empty the bag first?[/quote]
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danq said:

[quote author=Chad link=1210116638/0#1 date=1210116953]Yep, it's just air pollutants. There's really nothing else to it. IE, if you shine a laser in a vacuum, you won't see a beam at all.

Click to expand...

hey, I just tried this - your vacuum must be cleaner than mine. Or was I supposed to empty the bag first?[/quote]
HAH dude Hilarious 1+

the green also scatters much more than the red. this is why the sky is blue. well, actually its violet, but our eyes are much more sensitive to blue so thats what we end up seeing. without going into too much detail, the bluer the light, the more the light will be Rayleigh scattered off of the air particles (dust and other contaminants as well) and the more visible the beam will be. After you take into account the human eye's response to the different colors, green wins out as the brightest. I have a <5mW greenie and a 488nm argon running at about 6mW and I can see the green a bit more clearly.

climbak said:

the green also scatters much more than the red. this is why the sky is blue. well, actually its violet, but our eyes are much more sensitive to blue so thats what we end up seeing. without going into too much detail, the bluer the light, the more the light will be Rayleigh scattered  off of the air particles (dust and other contaminants as well) and the more visible the beam will be. After you take into account the human eye's response to the different colors, green wins out as the brightest. I have a <5mW greenie and a 488nm argon running at about 6mW and I can see the green a bit more clearly.

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i really didnt get it but i'm sure its VERY interesting, could you explain this better? is rayleigh a pattern or sth? what does scatter have to do with the sky being violet?

nikokapo said:

i really didnt get it but i'm sure its VERY interesting, could you explain this better? is rayleigh a pattern or sth? what does scatter have to do with the sky being violet?

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*sigh* it really isn't that complicated... don't you remember from school?

Roses are Red
Violets are Blue
You'd better Scatter
or the Sky will get You!

Now, of course we adults know that the above is really Allegory:*
the red roses refer to Henny Penny, the little Red hen; "violets" is actually Violence, the blue representing bruises received therefrom; and the Sky will get you - thus the need to scatter.


* Allegory: a scary tale about environmental devestation, a loss in environmental quality brought to our attention by a former politician who has removed his vest. Be sure to distinguish from devastation, which is loss of environmental volume.

nikokapo said:

is rayleigh a pattern or sth?

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I'm sorry, forgot to answer that part of your question.

Rayleigh is indeed a pattern - of behavior. It's actually spelled "Raleigh", and was named after Sir Walt (a lot of things have been named since Walt was born). It refers to the practice of covering puddles with obscuring devices, usually made of absorbent fabric - for the purpose of avoiding environmental scatter (which see above). This behavior - cloaking reality in the belief it will avoid catastrophy - is often observed in classes of individuals who engage - perversely - in naming those with a clearer grasp of reality as "chicken littles" (see above post).

haha i was unaware good old Raleigh also had a behavior pattern after him.

Rayleigh scattering is due to the interaction of the light with basically very small particles. Without going into all the math or anything, the shorter the wavelength of light, the more it will be scattered. Because violet light has a shorter wavelength, than say blue, it is scattered more, thus the sky is actually purple. However, our eyes are much much more responsive to blue than to violet so we end up seeing the blue. There is a lot more to it as well such as absorption and stuff, but that could take a long time to explain. If you want me to I can, just have to wait till I am finished with my last bit of school work. Rayleigh scattering also is one reason why the sunset is red. The red light is not scattered as much and so, for lack of a better term, cuts through the atmosphere more easily, while the bluer light is scattered and less of it reaches us. Is that a bit more clear, or did I just make it worse? It's late and my brain is a tad fried so I may very well have just been speaking in jibberish.

Green, yellow and Blue lasers have the most visible beams per mw. Green is the closest to your eye's peak sensitivity which is at around 555nm. Yellow comes in close at 594nm. Blue isn't very visible in the light but after your eyes adjust to the darkness it becomes much more visible. This is because your night vision peak sensitivity moves around 30mw closer to blue/violet. In a dark room a blue laser will look approximately 2/3 as bright as a green.

But Rayleigh scattering works only with really tiny particles, like air particles, comparable to the wavelength.In foggy or dusty environments , Mie scattering takes over , since fog/dust/smoke particles are way bigger than air particles.And Mie scattering is almost wavelength independent, so you don't have to worry about some lasers being scattered more in foggy environments.Is this right? :-/