I know that to see the beam of a laser the photons have to be reflected by something. Are there lasers so powerful that you can see them in nothing but air?
Well, as with every physics question there is always a slightly different “perfect” world (ie no air friction) in this case the perfect world (in which I belive the OP is refering to) would be NO impurities in the medium that the photons are traveling in, this would mean that there would be nothing for them to reflect off of and even the strongest laser or light source would not be seen in said medium.
In laymans terms: no.
Formerly known as Nec3f on the AOL SDMB
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- How visible it is also depends on where you are viewing it from. If you hold a laser pointer in front of your eye and to one side, pointing away(!), you can see it for some distance in fog and dust but someone standing beside you may just barely be able to see it. Someone standing ten feet to either side of you won’t see it at all. - MC
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In Tom Clancy’s book Cardinal of the Kremlin, the author goes into detail about fictional Star Wars programs on both sides. The Russians break through on the power front, able to pump a laser out in a straight line for many miles; the U.S. breakthrough is in targetting, where they can destroy balloons over Australia in a test. The Americans haven’t solved the power issue, and the Russians can’t target properly.</P>
Regarding the power problem, Clancy mentions thermal bloom, which is caused by the diffraction of the laser in the atmosphere, causing the laser to lose coherence. The Russians solve this problem by increasing the power; in effect, he says, they’re drilling a hole through the atmosphere, and the bit has to be spinning fast enough to prevent diffraction.</P>
How accurate is this, regarding the physics of lasers? Clancy is Mr. Technothriller of the eighties, so I assume there’s some merit to it. Still, trusting the man who made his fictional proxy president of the U.S. because a Japanese 747 pilot crashes his airliner into the capitol building while both houses are meeting to confirm him as vice president (Debt of Honor), well, it’s a bit much for me.</P>
CAUTION: Do not look directly into laser beam with remaining good eye!
Well, the thermal bloom thing sounds like it could be a simplification of a real idea, but it sounds kinda questionable. Increasing power wouldn’t prevent diffraction. If you lose say 20% of your beam to diffraction, you have 80% left. If you increase the power, that 80% is gonna be bigger. Not a real deep concept. 
To decrease diffraction, you’d need to raise the frequency. But a higher frequency would suffer from greater “scattering” (which I think would cause even greater losses). Incidientally, “scattering” is what makes a laser visible.
I don’t think there is a laser at today’s tech that would be visible today on a clear, dry day. A laser of that power would be a real hazard, as it could undoubtedly knock birds, planes and mebbe even Superman out of the sky.
If Stephmon were here, he could give the definitive answer.
Speaking of Clancy, Opal had me read a bit from The Hunt for Red October because she wanted my opinion on the physics of it. Let’s just say my appraisal wasn’t that great. He had an elaborate description of an issue with superheated water (he really made it sound like he knew what he was talking about), but the situation he described was in fact physically impossible (liquid water at temps well above the critical temp for water).
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- I don’t know the exact technical answer why, but as a laser’s power output is increased, it has to discharge in a shorter period of time. The most powerful lasers constructed fire millions of watts of energy, but only for a very small amount of time - <1/100,000 of a second. -And in some cases, they burn up critical components every shot and have to be rebuilt.
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- I have a book somewhere that says that it is possible to build a satellite that can shoot enough laser power to destroy a missile in boost phase. The practical problems make it unfavorable to do so, however. The first problem is lasers burning up when used at high power levels. The authors said this is solved by linking a bunch of laser “cores” into a chain, like machine gun belt ammo. Each core is fired once (which destroys it) and then essentially discarded, because there’s no way to fix it. The second problem was the number of targets - in only 60-120 seconds, tens of thousands of missiles would lift off and it wasn’t thought that hitting an effective number was possible. There is no proven method for aiming at so distant a target. (Remember, the US and Russia had enough nukes to destroy everything twenty or fifty or however-many times over. If just 5% of those got through, most of us would still be toast.) The third problem was cost - even a single trial satellite was considered much too expensive, even by Pentagon standards. - A laser that could destroy ground targets from space was considered too unreliable - it might work on clear days (if it could hit its target), but certainly wouldn’t work on coudy or hazy days.
- This book is a few years old, so anything here might not be true now.
- Sweet dreams! - MC
Undead Dude writes, “but the situation he described was in fact physically impossible (liquid water at temps well above the critical temp for water).”
Interesting factoid for the day:
It’s certainly uncommon, but it is possible to heat water beyond its boiling point (for a given pressure) without having it boil.
I don’t know what Clancy was talking about specifically, so I’m not defending what he wrote. It’s uncommon to see truely superheated water in day to day life, but its possible to make this happen if you try. When water is heated to its boiling point (about 100C at sea level), the phase transition to steam is prompted by what are called “nucleation sites”, which usually are imperfections in the container the water is being heated in, or impurities in the water, or regions of uneaven heating. In most day to day situations, the phase change does happen at the expected temperature.
But interestingly enough, if water is heated in the right sort of way (commonly via microwaves), in a very clean and smooth glass container, it is possible for it to exceed its boiling point without actually boiling. This is a fragile state for it to be in - bumping the container or adding some fine particles to act as nucleation sites will then cause the water to start boiling vigorously. It’s almost impossible to achieve this when heating water on a stove, due to the convection currents set up by the uneven heating element.
It’s hard, but not impossible, to do this in a household microwave oven. Some people have done it by accident; they remove a cup of non-boiling water from the microwave, do something that adds nucleation sites (such as add a powered drink mix), and suddenly the water starts boiling madly!
k0myers
That depends on how you define “air.” The N2 and O2 dont interact with visible light, so the wont relect a visible laser. Water vapor and dust will reflect visible light, so depending on the beam power density, quite a bit of light could be reflected.
If a 50 micon dust mote hits a 1mm^2 beam of 1watt laser, theres going to be ~3mw of reflected power. Thats about as bright as an ordinary hand held laser pointer.
Back when I worked in a class 10 clean room, we had some equipment that used a 400mw green diode laser, and the beam was DEFINITLY visible.
Na, na, na. I did’t say anything about water heated above the normal boiling point for 1 atm. Of course liquid water can be kept at a temperature higher than 100c, which anyone who has used a pressure cooker, or who has worked with hot water heaters can observe.
I said that Clancy described liquid water above the critical temperature for water.
The critical temperature for water is about 374c. Liquid water above this temperature at any pressure is impossible. The normal thermal kinetic energy in each molecule is just too great for liquid bonds to form.
Undead Dude says, “I did’t say anything about water heated above the normal boiling point for 1 atm.”
You’re right. My mistake, sorry!
I still think it was an interesting factoid 
k0myers
Good point. So in “real world” air, a lot of lasers would probably be a little bit visible at night.
I do remember reading that the US Military had developed a “Laser Cannon”, that would be used to blind many troops ( assumedly opposing troops ) at once. It would fire an intense shot of laser light, towards the opposition. I would think that while it would be the last thing they would ever see, the unfortunate victims would…see the light one last time? No?
Typer
Well, certainly a laser is visible if you look into it. I think the question was meant to be about viewing a laser from “the side”, or at least somewhere not in the beam.
Those soldiers might be temporarily blinded, but the laser would probably not cause total, permanent blindness. It would likely cause some permanent damage, but for it to cause total blindness, it would either need to be really powerful (killing the soldier), or the beam would need to be so large as to encompass the soldier’s entire view.
In La Jolla, there is a tall building from which they shoot two laser beams into the night sky simply for the artistic value. They are green, and definitely visible from miles away. (On a clear night they can be seen faintly in Del Mar, which is over 10 miles away.) When I was in school, they used to pass directly over my apartment. I always meant to try to fly a kite through them, but I never got around to it. Very likely that’s just as well.
I haven’t seen the green lasers over La Jolla in a while. I’m not sure if they’re still doing it or not. From what I understand, it was a real pain in the rear to maintain them… I think Qualcomm was doing it, and now that they have their name on the stadium they may feel that’s enough publicity.
The timing also corresponded vaguely with the Marines taking over Miramar NAS just a few miles south and east, which also might have had something to do with it.
UndeadDude wrote:
Let’s say I have a pressure cooker, filled to the top with water, and no air space. Let’s say I heat it to 400c, and the cooker is strong enough to still be intact. The water has no place to expand to. It has the same volume as before. Are you saying that those molecules are a super-pressurized gas, rather than a liquid? If so, can you explain the difference between a super-pressurized gas and a liquid?
I am not trying to troll or anything like that. I just never heard of Critical Temperature before. Thanks.
Yep, pretty much.
If so, can you explain the difference between a super-pressurized gas and a liquid? – Keeves
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There wouldn’t be anything special about this gas state, other than that it would be extremely dense. I’m not sure if it would be visible. Perhaps it would be. Other than that, it’s just a plain old gas.
In the case you described, I’m not sure if a liquid-vapor boundary would ever form. I think it probably would. I have watched a film of water with a small vapor layer above it reaching its critical temperature. It’s kinda trippy. When it hits the critical temperature, no “boiling” occurs, in the sense that there are no bubbles. The liquid just kinda drops out. It looks like someone drained the container very quickly. When I saw it, the process seemed to take about one second. Afterwards, it appeared to be invisible, but watching it on a film didn’t give me much of a chance for inspection.
This is all from my general knowledge of science:
Pure, dry air is mostly nitrogen, about 80%. Then there’s oxygen, carbon dioxide, methane, and lots of trace gasses. (And to be fair, normal air does have water vapor, too.)
Most photons of the electromagnetic spectrum, as a general rule, easily pass through a gas. There are some exceptions. Some EM waves have a wavelength that are the same size of the gas molecules. In this case, they have a hard time passing by without being reflected or scattered (photons with larger or smaller wavelengths than the surrounding gas mostly ‘get around’ the gas molecules).
For example, deadly levels of UV radiation are reflected by ozone in the upper atmosphere. The blue light from the sun is reflected/scattered by nitrogen, and that is what makes the sky look blue.
Now, even though photons with larger or smaller wavelengths generally get by, they can still have head-on collisions with any gas molecule, and be reflected or scattered.
So, any laser of any wavelength, if strong enough, will be bouncing enough photons into the ambient air molecules to become a ‘visible beam’.
And, I have seen a picture of a laser beam focused by a lense onto a single point in the air. At that point, there was enough photonic energy bouncing into the air, that the air was glowing hot.
Peace.
Even in the absence of dust, vapor, etc to reflect light from a laser beam toward a viewer, a sufficiently powerful laser beam will ionize the air it’s traveling through, creating a visible beam similar to a lightning bolt.
Above the critical temperature of a liquid, it becomes a “super-critical fluid”, which technically cannot be considered either a gas or a liquid. The Encyclopedia Britannica has ample information on this subject.