From: gmartin@zia.aoc.nrao.edu (George Martin)
Newsgroups: sci.skeptic,sci.physics,alt.folklore.science
Subject: Re: The dangers in microwaved food...
Date: 26 Aug 92 03:02:07 GMT
Organization: NRAO

In article <1992Aug22.075458.14172@murdoch.acc.Virginia.EDU> crb7q@kelvin.seas.Virginia.EDU (Cameron Randale Bass) writes:
>In article <22AUG199201293340@rigel.tamu.edu> elo9246@rigel.tamu.edu (LaNelle Ohlhausen) writes:
>>In article <1992Aug21.015046.12425@cs.mun.ca>, paulk@garfield.cs.mun.ca (Paul E. King) writes...
>>>I believe that theory has it that microwaves act on food by heating its
>>>water content. The wavelength of these waves are short enough that they
>>>are said to cause vibrations in the water molecules. This vibration
>>>causes the heat. 
>>
>>Small correction:
>>
>>The wavelengths of microwave radiation are appropriate to cause *rotation*, 
>>not vibration, of water molecules.  And these are in fact quite long
>>wavelengths in comparison with most familiar radiation.  The rotational
>>frequency for water is somewhere around 10 cm^{-1} (I don't have the exact 
                                             ^^^^^^^
>>value here, but can look it up on request), which implies a
>>wavelength of about 0.1 cm.  For comparison, infrared (heat)
>
>    Small correction:  The wavelength of your typical microwave oven
>    is about 12 cm.  Also, we are exciting no rotational resonance in 
>    water.  It is simply the field wacking the dipole moment around
>    (how's that for a rigourous argument?).  Also, frequency is 
>    measured in Hertz or 1/sec or cycles per second, rarely in cm.  
>    For the typical microwave, the relevant frequency is 2.45 GHz.

One more small correction or nit pick as you please.  A standard unit
in spectroscopy is inverse centimeters often refered to as wave
number.  The formal unit name, I think, is Kaysers.  Multiplying the wave
number by c in cm/sec gives you Hz of course.

>    You are correct if you are saying that water has a resonance at
>    around 0.75 cm (somewhere around 40 GHz, I'm too lazy to look 
>    it up right now).  However, that is wavelength
>    and this particular one (0.1 cm) is not relevant to discussion of 
>    commercial or home microwave ovens.

There is a very well known transition, I think it is rotational, which
makes a maser in astronomical conditions at about 22.2233345 GHz.  The
transition, whether or not conditions causes it to be a maser, has nothing
to do with resonance.

For those wondering about the terms rotation and vibration connected to
spectroscopy.  If you remember the tinker toy models of molecules,
picture the three orthonganal planes in which such a model can rotate.
Real molecules in free space do rotate.  But the angular momentum of the
rotations is quantized with all sort of selection rules which give rise
to spectral lines which can be observed in radio astronomy if the density
of the molecule is great enough.

Now instead of the "rigid rods" connecting the tinker toy balls making
molecules, think of springs, in particular quantum mechanical springs.
Thus the atoms which make up a molecule can be thought of as a bunch
of coupled harmonic oscillators.  And again things are quantized with
selction rules.  Vibrational transitions are typically in the infrared.
Of course there is also coupling between the rotational and vibrational
states of a molecule.
-- 
George Martin
Systems Analyst                     NRAO/VLA                  Socorro NM
Internet: gmartin@something.aoc.nrao.edu
"Skating away on the thin ice of the new day."  (Ian Anderson)
Newsgroups: sci.skeptic