Are there (fairly small, compact) lasers that are powerful enough that they can appear as a straight line in the air without the need for something to fog up the air to make their beam visible, or is that not how it works?
Images in the Wikipedia article on laser guide stars, especially the last one, show that you can see some lasers in clear air.
If you want a laser to be visible, it has to be of a wavelength that the human eye can see (so no infra-red lasers) and there has to be something for the light to scatter off of. Fog or smoke definitely helps, but the air itself will scatter some light. After all, that’s why the sky looks blue.
And just like the sky, and for the same basic reason, lasers that are more towards the blue end of the spectrum (blue and green lasers) will be much more visible in air.
Google “Rayleigh scattering” for more info on why the sky is blue and why blue and green lasers will be more visible.
Also, important tip when working with lasers, be careful not to point the beam directly into your remaining eye.
A powerful enough laser will ionize the air, creating a visible flash. No scattering needed.
Does this mean that weapons grade (megawatt plus) visible wavelength lasers have a clearly visible beam?
Now I am wondering: is this scattering enough to blind people standing behind or to the side of the laser when it fires?
The Navy has a plan to eventually develop and deploy missile defense laser systems on all it’s warships. I am wondering if deck crew members would be blinded if they happened to be looking the wrong way when the weapon fires.
I imagine anyone working with such weapons would have some sort of visor or goggles incorporating an optical filter tuned to the wavelengths used, to prevent just such things. I know I’ve seen patents for pilot helmet visors that incorporate such filters, to protect the pilot from laser-based dazzling attacks.
I think the energy densities need to be much, much higher. Terrawatt or more.
I saw a demo with a TEA laser, where they fired pulses at a parabolic mirror, and the light ionized the air at the focal point, creating a sharp “crack” and a bright flash.
BTW, this phenomenon limits the power of laser weapons. You want your energy to hit the target, not to be absorbed in the air.
You could say the same thing about bullets.
As mentioned, for all but the most exotic equipment, there needs to be particles to scatter the beam to be visible. The angle plays a role, too. Seeing a beam from a 90 degree or perpendicular angle requires more visible light and/or more particles than being at an angle closer to the axis of the beam and especially within a few degrees of the termination path. Things like photons tend to glance off particles rather than bounce at a hard, right dogleg.
And it’s not just lasers. Event lighting usually needs fog or haze machines to be able to see the beams from conventional lighting. I’ve heard this called beam effects. The visible beam is the desired outcome rather than illuminating a particular item where the beam terminates. Similarly, you don’t want haze if fixed illumination is the goal like a stage spotlight or a video projector. Here’s a good example: youtube. I’m mobile and might have goofed the link so skip to one minute in. Here, I’m guessing the hazers/foggers got a nice assist from the fireworks’ smoke.
That’s my absolute favorite workplace safety sign ever: “Caution: do not look at laser with remaining eye.”
I love it because it’s funny, but also because I believe it’s a lot more effective as a safety device than the “real” laser safety signs.
The best laser safety message was one I saw on the door at a lab at Goddard SFC.
They had an article from a medical journal taped to the door. The article described the traumatic eye injury suffered by someone who caught a laser reflection in their eye (it was powerful enough that they heard a “pop”). It had photos of the burned spots on the retina. At the end of the article, someone had written “The laser in this room is 10,000x more powerful than the one in this article. Wear your safety glasses."
Of course.
See here for an example of the protective gear the sailors had to wear when loading the big guns on WWII battleships. Covered up more than a Muslim terrorist in a burka.
Recall that among various directed-energy ABM scenarios, excluding ithose that involve physically disrupting sensor technology, the energy on target, and choice of target area, must be enough only to cause enough deformation of surface structure to cause buckling. Ie, not vaporizing the thing.
It’s true that dimmer beams may be able to disable missiles through mechanisms like you describe. On the other hand, the brighter the system, the better it works through fog and mist and other imperfect conditions, and the shorter the engagement window required for firing at targets such as hypersonic wave skimming missiles. Or firing at high altitude ABM reentry vehicles that are armored against laser damage.
It’s theoretically possible for these laser systems to be scaled to the point that a naval warship would be able to engage aircraft and satellites and basically any target within line of sight. Obviously, a far brighter beam would be needed, as well as much better optics and sensors for such a shot to be possible.
I read an article about a retina that had been sliced “beautifully” by a laser.
But how do safety glasses protect your eyes from a laser?
We’ve had discussions here about the effect of pointing laser pointers at aircraft, but the discussion was limited by the absence of anyone with an actual understanding of the physics involved.
How much does scattering affect divergence / beam spreading?
They have dyes or coatings that absorb the particular frequency laser that you are working with. Now, if you are dealing with a megawatt laser, off-the-shelf goggles are probably going to melt when hit by the beam. But, for normal lasers in the sub-KW range, googles are sufficient protection. Better than stopping the beam with your retina.
so star wars type of blasters wont be possible one day ?
Yes, absolutely.