Re: The sound of a laser.

From: Randy Poe (poespam-trap_at_yahoo.com)
Date: 08/20/04 

Date: 20 Aug 2004 08:04:04 -0700

"David Prokopetz" <sirbob@penguinking.com> wrote in message news:<2ol510Fbmdq4U1@uni-berlin.de>...
> Hi.
>
> Physics newbie question here: what does a laser sound like? Some sources
> I've read claim that a sufficiently powerful laser would ionise the air
> along its path and produce a "crackle" or "pop" noise,

Biggest laser I ever worked with was 2 W, so I have no
first hand knowledge. What you're talking about is usually associated
with high voltages, such as a lightning bolt, an electric discharge,
or the corona around a high-voltage line. That wouldn't be a
mechanism for a laser to ionize air.

However, if it made the air hot, that *would* be a mechanism for
this sort of thing. So the question is, under what circumstances
would laser heat the air along its path?

You can see a powerful laser beam from the side because of
scattering off dust in the air. I could imagine a powerful
enough laser heating the dust enough to boil it. Hence, a
crackling sound.

> while others claim
> that regardless of power, a laser has to be of a particular colour to ionise
> the air and thus produce a sound

Not true. However, dust and air molecules, like everything else,
absorb different wavelengths differently. Black dust absorbs
most visible wavelengths, so it would get hotter. Air is
transparent in the visible, but not 100% transparent. so
the air along the path is going to get microscopically hotter.

However, obviously you'll put more energy into the air, making
it hotter, if you choose a wavelength where it absorbs. There
are probably some good choices in the infrared.

I don't have anything like an absorption spectrum for nitrogen
handy, but that would tell you which wavelengths are preferentially
absorbed.

            - Randy

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