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# Iridescent air

 (+2) [vote for, against]

Suggested by an anno. by [bigsleep] on "Iridescent Water".

Sound is simply compression waves travelling through a medium. In the case of a gas, the degree of compression is quite small, but not zero.

Small changes in air density cause light to be refracted - this is commonly seen as heat-shimmer, where the density changes happen as a result of heating.

Sound waves, therefore, ought to impose a sort of refractive fingerprint on light. Normally, these effects would be non-noticeable, because the degree of compression is very small, and hence the deflections of light would be minuscule.

However, what if you could generate sound in air with a wavelength comparable to that of visible light (o.t.o.o. 1µm)? This should do something weird, akin to iridescence. Iridescence happens when some periodic structure (like ridges on a beetle's back) matches the wavelength of a particular colour of light. A wavelength of 1µm in air corresponds to 300mHz.

So, to wind up this rambling. Is there any way in which a gas could be made iridiscentish by the application of ludicrously high-frequency sound?

 — MaxwellBuchanan, Apr 23 2012

Making sound waves visible with light http://blog.modernm...ound-waves-picture/
Like this? [Alterother, Apr 23 2012]

Visible sound Visible_20sound
We tried to find a good solution to this before [hippo, Apr 24 2012]

I really do like this picture http://apod.nasa.gov/apod/ap010221.html
[hippo, Apr 24 2012]

The Prandtl-Glauert singularity http://www.fluids.e.../conden/pg_sing.htm
[hippo, Apr 24 2012]

Discussion of maximum sound frequncy in air http://www.physicsf...x.php/t-144509.html

 Tesla or somebody frigged around with this. I read about it in a book, but I'll see if I can find anything linkable.

Yep, found it <seelink>. Not Tesla, though.
 — Alterother, Apr 23 2012

//Tesla or somebody// The problem with Tesla is that he was half genius, half charlatan. Still, I'll be interested in the link if you can find it.
 — MaxwellBuchanan, Apr 23 2012

I have also personally witnessed air distortion in front of giant subwoofers at heavy metal concerts. There are probably dozens of videos of that effect on the interwebs.
 — Alterother, Apr 23 2012

That's encouraging.
 — MaxwellBuchanan, Apr 23 2012

 Well, I'm not sure if any of this is what you're getting at, but it's what I have to contribute.

In fact, it's a lot more than I usually bring to the table...
 — Alterother, Apr 23 2012

 300 MHz would be about 1 metre, rather than 1µm, if you're talking about electromagnetic radiation, I'm afraid. I think that's the point [bigsleep] was making, rather discreetly.

Ah, a sound wavelength of 1µm = 340.29MHz at sea level.
 — UnaBubba, Apr 23 2012

 It's EM, but not as we see it, Jim.

You might pick it up on your iridescent UHF radio? Tactical military comms are often in this sort of range, I believe.
 — UnaBubba, Apr 23 2012



You are correct, sir.
 — Alterother, Apr 23 2012

You could add fine water droplets to the air. Then, at certain angles, it might be possible to observe a multi-hued iridescent effect, of light refraction.
 — UnaBubba, Apr 23 2012

<Bows. Wanders off, whistling Saltimbanco.>
 — UnaBubba, Apr 23 2012

 //300 MHz would be about 1 metre, rather than 1µm, if you're talking about electromagnetic radiation, I'm afraid.//

 Indeed it would, but I am not talking at all about electromagnetic radiation.

What I am saying is "create sound waves in air with a wavelength comparable to that of visible light, and see if the resulting refractive properties cause some sort of iridescentoid interference".
 — MaxwellBuchanan, Apr 24 2012

If the air is at a normal temperature and pressure and very humid - i.e. 100% humidity - then, as the sound wave travels through the air the low-pressure parts of the wave should take the air pressure below a point where it's able to hold so much humidity and the water will condense out into clouds, the thickness of which will be proportional to the sound's amplitude.

Also, see links. This would be cool, just because you'd be able to shout "My God! It's a Prandtl-Glauert singularity!!!"
 — hippo, Apr 24 2012

 //the water will condense out into clouds, the thickness of which will be proportional to the sound's amplitude.//

By thickness, I presume you mean density? One problem is that water droplets will need to be very tiny (o.t.o.o. 1µm) in order to create the necessary layered cloud structure for iridescence.
 — MaxwellBuchanan, Apr 24 2012

Yes, I mean density. Anyway it doesn't matter whether it shows iridescence - your audience will be too impressed with hearing you shout "My God! It's a Prandtl-Glauert singularity!!!" to notice.
 — hippo, Apr 24 2012

 //your audience will be too impressed// They always are.

 Oh, wait - _IM_pressed?

// Prandtl-Glauert singularity!// As long as you can find me a large enough volume of perfect inviscid gas, we're in business.
 — MaxwellBuchanan, Apr 24 2012

I'm not sure what "inviscid" means, but we could try locking [8th of 7] in an airtight room and see what accumulates.
 — Alterother, Apr 24 2012

Inviscid = non viscous. Things like viscosity, and departures from the perfect gas laws, are what prevent Prandtl-Glauert singularities.
 — MaxwellBuchanan, Apr 24 2012

//_IM_pressed// - only if you're doing physics experiments for the navy.
 — hippo, Apr 24 2012

 Non-viscous... I probably could have figured that out, but I'm too busy with other words today. Deadline comin' up.

My plan probably won't work, then. There's not much about our Borg that's inviscid.
 — Alterother, Apr 24 2012

 // No. The highest (effective) frequency that can exist is linked to the mean free path. This at sea level is about 10^-7m. Hence the highest frequency (even loosly defined as sound) is 340e7 Hz. Even that is stretching a point.

 [link removed] gives a general account of attenuation. At 100kHz we have about 1800dB/km in fairly dry air. Attenuation on a simple model goes up as f^2. Range of 1000kHz therefore bein of the order of 10m (1.8dB/m). At 1MHz we have 10cm. At 10MHz 100 microns. In my book 100MHz+ can't really exist in air.

 - Ian Parker //

I can't independently vouch for everything said there, but it makes sense to me.
 — scad mientist, Apr 24 2012

 Thanks for the link, [scad]. The thread there says: //At 100kHz we have about 1800dB/km in fairly dry air. Attenuation on a simple model goes up asf^2. Range of 1000kHz therefore bein of the order of 10m (1.8dB/m). At 1MHz we have 10cm. At 10MHz 100 microns. In my book 100MHz+ can't really exist in air.//

So, it looks like 300MHz (corresponding to a 1µm wavelength) is out, but a wavelength of a few microns (<100MHz) should be producible over small (millimetre) distances. A few-micron spacing should still produce some iridescent phenomena.
 — MaxwellBuchanan, Apr 24 2012

As would salting the air with very fine particulates of glitter.
 — UnaBubba, Apr 24 2012

That's a good point - would this be an easier effect to create in a smoke-filled room?
 — hippo, Apr 25 2012

I suspect it might be, [hippo]. The exact conditions necessary to cause clean air to iridesce would probably be in a very narrow band, difficult to replicate.
 — UnaBubba, Apr 25 2012

//A wavelength of 1µm in air corresponds to 300mHz.//
300mHz in air is well over 1km.
 — AbsintheWithoutLeave, Apr 25 2012

 //300mHz in air is well over 1km.//

 300 mHz of what?

Light at 300MHz / 300,000,000 Hz = .9993082 metres in a vacuum or about .9990082 metres in air at STP
 — UnaBubba, Apr 25 2012

 //300mHz in air is well over 1km.//

 I fear you may have missed the point.

I am talking about _sound_ waves in air, _not_ _electromagnetic_ _waves_. Sound waves. Waves of sound.
 — MaxwellBuchanan, Apr 25 2012

A wave of sound in air with a frequency of 0.3Hz would have a wavelength of well over one kilometre, assuming the speed of sound is around 340 metres per second.
I don't believe I have missed any points.

It is worth pointing out that I work on devices where frequencies are specified over more than 30 octaves. I have to know where my shift-key is, and what difference it makes.
 — AbsintheWithoutLeave, Apr 25 2012

Ah, I assumed the m was a typo. Sorry.
 — UnaBubba, Apr 25 2012

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