I’m sure we’ve all (ok, most of us) been to concert where the spotlights produce a beam of light piercing through a cloud of smoke. For me, naturally I assumed it was from all the smoking of what have you. Now, I haven’t been to a concert in ages. (Heck, I don’t even know if smoking is allowed anymore.) But, recently, I attended a concert-like event for pre-teens in an indoor amphitheater where there was clearly no smoking allowed. Still, the spotlights were piercing through clouds of smoke (or fog), But what is the source of this? No fog machines were in use during the performance or any other type of smoke-generating theatrical special effects machines.
Could it be that a spotlight is so hot, there is actually a temperature gradient with the adjacent air to see a cloud of fog form from water moisture precipitating out into a cloud formation where the hotter air and cooler air meet? Also, I noticed the “fog clouds” stretch a good distance, although thinning with distance from the spotlight. I assume this is all part of the spotlight creating a gradient?
It’s just dust in the air. Air is filled with dust.
Fog is made of water droplets. You get fog when you cool down air below its dew point, so that water vapor can no longer remain as water vapor and become liquid droplets. Warming up air with a spotlight will do the exact opposite.
How do you know that there weren’t any fog machines? They are small and unobtrusive. Can be as simple as a bucket of water with a hunk of dry ice in it. I would think that they would be co-located with the spotlights for the greatest effect. Is it just that you didn’t see them, or that you have a reason to know this?
As far as the spotlight creating the fog, I do not think so. If it were cooling the air, it may create some sort of effect like that, but heating the air will just lower the humidity further, and mixing that back in with the air it came from would make no noticeable effect.
My WAG is that there were fog machines that you just did not notice.
The other thing is that fog occurs when water vapor condenses (usually on dust particles or something) out of the air. This happens when the air cools, not when the air is heated. To the extent that a spotlight might heat the air, it would prevent fog from forming, not cause it.
This is correct. If you see spotlights outdoors you will see the same thing. If you put on a flashlight in your house in a really, really dark room you will see the same thing. The effect is greater when the light is brighter, and more narrowly focused. Those spots they use for stage shows are incredibly bright.
I will yield to the general consensus, but I find it hard to believe it is dust. When dust is that thick, I can smell it. It has a distinct, unpleasant and choking kind of “smell”. Not an odor, but a “smell” like what I can only describe as “stale air”. Also, if it were dust, how come other spotlights (using LEDs only) lacked such clouds?
I agree - fog machines can be about the size of a standard video projector and are very controllable now (it used to be that they were either completely off, or belching conspicuous thick fog, but modern units can just emit a fine background mist). They’re really common in lighting design, especially if the design includes narrow spots that the designer wants people to perceive as beams.
I know what you’re talking about, but I think you’re misunderstanding the scale. We’re not saying the air is thick with dust (like a sealed room, or even a large warehouse). What you’re seeing is a very bright light shining through a deceptively large area. The light column is not only wider, but it’s also a LOT brighter than a normal light, and highly collimated*. Because of this, the amount of light reflecting off each dust particle is many times what you’d see with a flashlight.
the photons are moving roughly parallel to each other, which means that a beam of light doesn’t disperse as much (think laser vs. shotgun). Sunlight is highly collimated, whole most light bulbs and even flashlights are much less collimated.
I used to manage a “clean room” that was a Class 100,000. Meaning 100,000 dust particles per cubic foot. That’s the clean room, mind you, suitable for non critical spacecraft work. Unfiltered air varies tremendously, but counts in the millions per cubic foot are not out of the ordinary. And that air does not appear dirty in any way.
Anecdote: Shortly after our clean room went “clean”, we had an outbreak of flies apparently left behind during construction somehow. Researchers would find them on counter-tops each morning. They brought it up in a weekly meeting and I joked, “Hey, each one is only one particle, we still meet spec!”
Another anecdote: We had a large laser table that we wanted to photograph in action. The photographer visualized the laser beam bouncing around the various mirrors and lenses, etc. Of course you cannot see a red laser beam in a dust free area (this was a Class 10,000 clean room). Back then you could still smoke in buildings, so we got ciggies from a smoker and handed them out to a bunch of us crouched low around the table. Each of us had a section of beam assigned to keep “lit”. We puffed and blew for the required 20 second exposure, enough to make sure none of us ever wanted to smoke again.
The smoke lit the beam brilliantly, to the point it was uncomfortable to look at without goggles.