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In chemistry class recently, my teacher was telling us about how a microwave works. She says that the EM waves emitted by the microwave hit the water molecules in food and cause them to vibrate faster by constructive interference with their vibrational motion, and therefore cause an increase in heat energy.

My question on hearing this was, if EM waves heat water by superposition with its vibration, why don't we ever see it cooling water with destructive superposition?

Is my teacher's explanation wrong in some way or am I missing a reason for definite constructive superposition?

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    $\begingroup$ If you had a sample of warm water that could be kept liquid inside a vacuum, it would cool by emission of microwaves. $\endgroup$ – ProfRob Nov 9 '16 at 6:43
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Cooling by such a procedure is actually the same technique used in laser cooling (more or less). You bombard the molecules or atoms with EM radiation, whose frequency is different from the natural frequency of oscillations of those ions, which as a result causes loss of heat in such systems.

But the important issue is that the frequency of this radiation has to be different from the natural frequency of the molecules/atoms; different in a way to actually cause destructive interference. For water, infrared and microwave actually happen to be close to the natural frequency of water molecules, and hence you cannot cool water using them. So if you use a different frequency radiation, it should be possible to cool water (i.e., crudely laser cooling).

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I think your teacher's explanation is wrong.

Microwaves heat food following this mechanism:

  • make the dipoles in your food rotate and change directions very quickly
  • particles in your food moving around will collide with others
  • these collisions transfer kinetic energy
  • the global kinetic energy of your food is increased, this is perceived as heat

There is no interference mechanism here.

More information here: https://en.wikipedia.org/wiki/Dielectric_heating

Laser cooling is a completely different mechanism, one of them involves optically pumping atoms to a lower energy state and have them climb the potential repeatedly. See https://en.wikipedia.org/wiki/Sisyphus_cooling for more information.

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    $\begingroup$ Just to be clear, Sisyphus cooling is a very specific mechanism, and it is only used in the final stages when you need particularly cold atoms. Generally, laser cooling is understood (in the absence of other qualifiers) to mean Doppler cooling, which is much simpler. $\endgroup$ – Emilio Pisanty Nov 9 '16 at 13:03

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