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I was told once that microwaves work by exciting water molecules in food. Also that this worked because the frequency in the microwave was the same as that in the bond between Oxygen and Hydrogen in the water molecule. This similar frequency match lead to the molecule being energised, releasing heat into the food.

My question is - how do microwaves work? Does any of this 'hearsay' have any basis in scientific fact?

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en.wikipedia.org/wiki/Dielectric_heating, maybe this helps... –  udiboy1209 Jul 22 '13 at 8:08
    
I have also heard that molecules can only absorb certain frequencies of light, depending on the wave function of their electrons, but unfortunately I cannot find any reference right now. –  udiboy1209 Jul 22 '13 at 8:17
    
Thanks @udiboy - that suggests it is not a direct correspondence of frequency, but the alignment behaviour of a dipole bond in an electromagnetic field - and an oscillating electromagnetic field causes the dipole-containing molecules to rotate releasing heat. –  hawkeye Jul 22 '13 at 10:03
    
But that still doesn't explain why it only heats up with microwaves. Oscillations in the electric field are caused by any EM radiation. I'm waiting for someone else to answer this. –  udiboy1209 Jul 22 '13 at 11:25

1 Answer 1

up vote 7 down vote accepted

Your question comes down to whether the EM absorption is a resonant process or not, where resonant means it corresponds to the energy of some excitation of the water molecule. The answer is that it is not a resonant process. Microwave ovens operate at 2.45GHz but the lowest energy transitions of water molecules are rotational transitions, which have energies in the 100GHz to 1THz range. The energy of the photons in a microwave oven are too low for any resonant absorption.

Google for details of the rotational spectrum of water. I found examples here and here.

The EM radiation from the oven makes dipolar molecules within it line up with the electric field. As the field oscillates the water molecules change direction (at 2.45GHz). In liquid water the molecules interact strongly and exchange energy with each other, so the energy of the flipping motion gets transferred to translational energy of the water molecules i.e. heat. Because this is not a resonant process changing the microwave energy by small amounts (up to an order of magnitude) won't make a lot of difference to the heating. As some of the comments have mentioned, this process is called dielectric heating.

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Awesome info on resonant process re EM absorption. –  hawkeye Jul 22 '13 at 14:15
    
@JohnRennie, So why doesn't dielectric heating happen with visible light? or any other frequency for that matter? –  udiboy1209 Jul 22 '13 at 17:36
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@udiboy: dielectric heating requires a physical movement of the molecule to line it up with the electric field. At optical frequencies the field is oscillating much faster than the molecules can move to respond to it, so energy cannot be dissipated by molecular motions. –  John Rennie Jul 22 '13 at 18:32
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@udiboy: Having said this, there are lots of other ways of absorbing EM radiation. My microwave produces about 800W. I guarantee you that if you shine an 800W lamp at a potato it will heat up just as fast as it does in an 800W microwave. It's just that the absorption mechanism will be different. –  John Rennie Jul 22 '13 at 18:35
    
My memory of the resonance rotational frequencies for water were a lot lower than you asserted. Searching for published material I find: www.jmpee.org/JMPEE_PDFs/28.../JMPEE-Vol28-4-Pg196-Roberts.pdf‎ which says that water liquid phase resonances exist at 17 GHz and water vapor resonance were measured at 23 GHz. I also question your reading of the first and second citations which seems to indicate much lower frequencies associated with transitions. Several absorption lines are in the 20 GHz area. (See p 60 of the second citation.) –  DWin Aug 29 '13 at 23:59

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