Can class IIIB lasers show a visible beam without smoke or fog?

  • Level: High School 
  • Thread starter Thread starter Anzas
  • Start date Start date
  • Tags Tags
    Pictures
Join the discussion
Registration is free. Start your own thread to ask a follow-up.
Sign up Log in
21 replies · 5K views
Anzas
Messages
87
Reaction score
0
http://www.megalaser.com/gallery.htm

they sell class IIIB lasers are these pictures genuine?
can class IIIB lasers really show a beam without some medium like smoke?
 
Last edited by a moderator:
Physics news on Phys.org

Discussion

One of the pictures claims to show the beam without fog. I am holding a class III laser in my hand right now (between keystrokes!) and I see no beam - just a bright spot on the wall! If there is, in fact, no fog then they have substituted something else like smoke or talcum powder dispersed in air.
 
Bottom of the page. "*Please bear in mind that the camera exposes the beam in a different way than the naked eye."
 
You can't see the beam without something in the air to reflect light.
 
The pictures look to have been photoshopped. They are certainly not real depictions of lasers in action.

Claude.
 
Yah the camera might have amplified it. Most cameras these days can adapt to low-light situations and one consequence might be that it makes a very very very very dim image (such as a laser going through the air) show up when a human would be unable to perceive it.
 
omg those pictures were funny!

"Its quite simply the most powerful laser in the world"'

I'm still wondering how two AAA batteries supplies 15 Megawatts?

Too bad some people would buy them.
 
Well i assume they mean milliwatts since normal handheld lasers are in the milliwatt range
 
Yes, of course. But the site clearly capitalizes the "m" making it MW Megawatts. The site is a fraud anyway, if a laser manufacturer made the site, it would have been mW.
 
I have a green laser pointer and its beam is very visible against a dark background. As I understand it, the reason we're able to see the green beam and not the beams of the more common red laser pointers is due to the fact that our eyes are much more sensitive to green light. Amateur astronomers use the green pointers for locating objects in the night sky because they're so visible. The photos posted here exaggerate the effect, but it is real.
 
20 Watt Copper vapour lasers produce a noticable green glow (within the cavity where the power densities are high), but it is quite faint.

When the site says MW, they may be referring to the irradiance, (which actually has units W/m^2). In any case, the poor presentation of the manufacturers specifications means that many of the claims are probably exaggerated.

In any case, the pictures are most certainly not real, and this in itself puts the credibility of the site very close to zero.

Claude.
 
I suspect that the pictures could be real, but taken with a lengthy exposure (perhaps several seconds).

According to Wikipedia, high intensity lasers can be visible in clean air by Raman scattering or Rayleigh scattering.

WiseGeek.com also suggests that green-beam lasers are visible in clean air.
 
Looks like a manipulation of exposure time, look at the 3rd or so picture.
 
I considered that they may have used a long exposure time, however it is very difficult to hold a laser steady over a second or so, usually you get a star wars-esque lightsabre effect, which is why I suspect the photos have been digitally altered.

I should also emphasise that the picture depicted in the wikipedia entry shows a clearly visible beam, however the beam is within the cavity where the energy density is typically much greater than the ouput beam (Also, I suspect the laser has substantially more power than the laser pointers if my previous experience with military research is anything to go by).

Typically a beam has to be in the realm of Watts to be visible in the air, and even then it is faint.

Claude.
 
Typically a beam has to be in the realm of Watts to be visible in the air, and even then it is faint.
Not that I doubt you :wink:, but would you have any supporting references?
 
Vaguely related popular culture fact:

In the James Bond movie 'Goldfinger', the laser beam (in the scene where Mr Bond is not expected to talk, but to die) was drawn onto the film with a marker pen, or some such.

The sparks flying off the steel bench that Sean Connery was strapped down to were created by a chap with an oxy-acetylene torch beneath the bench, working up to a chalk mark just beneath Mr Connery's crotch. The beads of sweat on the actor's face were real.

/OT
 
PeteSF said:
Not that I doubt you :wink:, but would you have any supporting references?

Not off-hand, this is from personal experience in working with Copper Vapour lasers. I guess one could calculate what the scattered intensity would be, but that's not an exercise I'm willing to undertake right now :wink: .

Claude.
 
Last edited:
80 mW? I hope he wears eye protection when he uses it, cause that can do some serious damage.

Claude.
 
Once skepticle like you all, I once firmly believed that you couldn't see a laser beam shining through ordinary air without some sort of material [extra] to help scatter the light, no matter what color of laser it is.
The my brother bought a <5 mW green laser pointer, and I have seen with my own eyes, yes, you can see the beam through normal air.
But there are some limitations. In a brightly lit room, you can only barely, if at all see the beam. If you go outside into the black night (or even in a dark room for that matter), the beam becomes clearly visible and you can see it extend hundreds of feet down the street.
But in no way is it anything like a star wars "light saber", or like the pictures on that web site. Now accidents will happen, and I am told, [as is true for any laser, it is a bad idea to get it in your eye], when the <5 mW green laser hits your eye, for a while afterward, everything looks very milky white.

The reason why it is so visible compared to a normal red laser? I am not exactly sure. I have heard to theory that human eyes are mor sensitive to green light than red, which could be part of the reason. And/or perhaps because the green (532 nm) laser has a shorter wavelength than a red (635 nm) laser, allowing it to bounch off smaller objects suspended in air?
 
Rayleigh scattering has a wavelength^-4 dependance, so a drop from 633 nm to 532 nm will result in a significant increase in scattered light. Also, the peak response of our eyes is in the yellow part of the spectrum (around 550 nm), so 532 nm is very close to the peak, whereas 633 nm is a fair way away.

It is not surprising that the beam looks bright in darkness, since our eyes are logarithmic detectors. In ordinary light though, the visibility of the beam is greatly reduced.

Claude.