Why a laser can be seen...

[Trashman]

I'm asking. On another forum, somebody said a laser's power doesn't have anything to do with it's visibility. I objected, using green lasers as an example, saying that lower powered ones can't be seen in day light, while higher powered ones can, thus proving power does affect visibility. Another person said lasers can only be seen, because their light is bouncing off dust and stuff in the atmosphere. That one I'm curious about. What if a laser was shot in a clean vacuum, with no dust for the light to bounce off of? Would we be able to see the laser, provided it had enough power and it was of the right wavelength? (lets just use the typical 532nm green laser for an example)

 

[Trashman]

What, no laser experts logging in? I know you're here!

Ok, then answer me this:

Are laser light waves similiar to radio waves in that the amplitude of the wave increases with more power, thus making it a wider/larger wave, and is that why the more powerful lasers have a thicker beam?

 

[Noctilucent]

I'm asking. On another forum, somebody said a laser's power doesn't have anything to do with it's visibility.

Untrue - presuming you mean output power, of course.

I objected, using green lasers as an example, saying that lower powered ones can't be seen in day light, while higher powered ones can, thus proving power does affect visibility.

Keep in mind that many of the 'higher powered' ones sold to consumers these days do also happen to be the green lasers you're referring to, while the 'lower power' ones tend to be the classic red lasers. The human eye is more sensitive to green than it is to red and can resolve greater detail at green wavelengths (hence the Bayer pattern of digital cameras having double the green photosites). Thus, the color affects visibility as well.

Another person said lasers can only be seen, because their light is bouncing off dust and stuff in the atmosphere.

Correct. Light is just photons. For you to see those photons, they have to reach your eye. The only way for the photons from a laser to eventually reach your eye is either by having the laser pointed at your eyes directly (uhhm... don't. seriously, do not do that.) or for the photons to bounce off of other surfaces and then to your eye.

In 'air', that tends to be dust particles, soot particles from traffic, water vapor, etc.

That one I'm curious about. What if a laser was shot in a clean vacuum, with no dust for the light to bounce off of? Would we be able to see the laser, provided it had enough power and it was of the right wavelength? (lets just use the typical 532nm green laser for an example)

No, regardless of the strength of the laser. It could be that crazy multimegawatt laser they use in high energy physics experiments and (if were actually visible light they used) you still wouldn't see a beam.

*however*

At some point you are going to create lasers that are so incredibly powerful - absolutely ridiculously powerful - that physics as you and I know it go right out the window, and the lasers can conceivable create matter out of their own energy.. when that happens, there's once again matter for the beam to reflect off of, and you would see the beam. Technically, though, it would no longer be a perfect vacuum environment as the matter would be the contaminant.
This is one of many recent articles on that topic:
http://www.gizmodo.com.au/2010/08/lasers-so-powerful-they-destroy-themselves/

I'm not a physicist, though - I deal more with biology and mineralogy - so don't ask for details as to the above process

Are laser light waves similiar to radio waves in that the amplitude of the wave increases with more power, thus making it a wider/larger wave, and is that why the more powerful lasers have a thicker beam?

No. Radio waves are only related to light in terms of wavelengths. Light itself is an oddball particle/wave dual thingymabob (did I mention I'm not a physicist) that you can't apply other electromagnetic field properties to so easily.
Most simply put - the more powerful lasers have a -brighter- beam simply because they emit more photons per given timeframe. The 'thickness' of the beam is entirely a function of the collimator being used. Lasers are often regarded as having a constant thickness.. but shine a laserpointer far enough (kilometers, not meters) and you'll see that they do typically diverge a little bit. Similarly, an appropriate assembly can converge the laser to just a tiny pinprick. This has been used to create a 3D display, for example, exhibited at Siggraph by turning the air into a plasma at the focal point of the laser.
http://www.youtube.com/watch?v=He2QTpelAjE

 

[Steve K]

What if a laser was shot in a clean vacuum, with no dust for the light to bounce off of? Would we be able to see the laser, provided it had enough power and it was of the right wavelength? (lets just use the typical 532nm green laser for an example)

consider the situation of a light source powered by a nuclear fusion reactor, and the reactor was thousands of times bigger than the earth....
Now take all of the power generated by that fusion, turn it into light, and let the light travel across space (which is a vacuum free of air and dust(mostly) and water vapor, etc.).

Would you be able to see that light as it goes through space, or would you have to wait until that light hit your eyes or reflected off of something?

Obviously, I'm talking about the Sun radiating light in all directions. We can only see the light when it hits out eyes (quit looking directly at the sun!) or when it reflects off of the grass, the clouds, the moon, etc.

If we see the full moon at night, we see light reflecting from the moon's surface, but we don't see a glow from all of the sunlight traveling past the earth and moon, even though there is an awful lot of power and energy in the light (much more than a laser).

this is just one way of thinking about the question...

regards,
Steve K.

 

[MikeAusC]

. . . and we see a blue sky where the sunlight has reflected off the atmosphere.

Look at photos of earth taken from space and you can see the atmosphere - but beyond that it's black, because there's nothing for teh sunlight to reflect off.

 

[wyager]

First-a brighter laser is more visible. In air, Rayleigh scattering makes the beam visible even without dust. Second, a more powerful laser does not have a higher amplitude-that's impossible. Even radio waves don't actually do that. Photons are always at a fixed magnitude relative to the frequency, more power=more photons. A 200mW red laser will have many more photons than a 200mW blue (but not more energy).

 

[PeaceOfMind]

No. Radio waves are only related to light in terms of wavelengths. Light itself is an oddball particle/wave dual thingymabob (did I mention I'm not a physicist) that you can't apply other electromagnetic field properties to so easily.

This isn't really correct. Radio waves and light waves are fundamentally the same thing (Nasa will back me up on that: http://imagine.gsfc.nasa.gov/docs/science/know_l1/emspectrum.html ).

The "oddball particle/wave" thing is called wave-particle duality. Radio waves can exhibit wave particle duality just as visible light can. The only difference is that light is a higher frequency than radio waves. Note that electromagnetic waves tend to behave more like particles at higher frequencies. Thus, light behaves more like a particle (photon) than a radio wave does, but there are no special rules for light vs radio waves (or vs gamma rays for that matter), and visible light isn't some oddball thing compared to other electromagnetic waves.

 

[CKOD]

First-a brighter laser is more visible. In air, Rayleigh scattering makes the beam visible even without dust. Second, a more powerful laser does not have a higher amplitude-that's impossible. Even radio waves don't actually do that. Photons are always at a fixed magnitude relative to the frequency, more power=more photons. A 200mW red laser will have many more photons than a 200mW blue (but not more energy).

I wanted to point this out too. The theoretical situation "in a vacuum" eliminates this, but if you replaced that with "In a super clean room" I.E. the purest air you could get, the scattering would make a laser visible, the closer to blue (like the sky) the laser is, the more it scatters.

 

[wyager]

I wanted to point this out too. The theoretical situation "in a vacuum" eliminates this, but if you replaced that with "In a super clean room" I.E. the purest air you could get, the scattering would make a laser visible, the closer to blue (like the sky) the laser is, the more it scatters.

Correct. Vacuum, no beam. Also, as you said the farther you move from red to violet in terms of beam color, the beam gets brighter and brighter compared to the dot. Even a 10mW 405nm has a (barely) visible beam at night. The dot is about as visible as a 1 or 2mW red laser. No 2mW red laser has a visible beam, even at night.

 

[Trashman]

Weird, I swear I already said thanks for the answers, and just came back to see if there were any more posts, and apparently, I never hit return, before, because my last post isn't here. So, thanks, guys, this is very helpful!