Probaly bad anology.
What i'm trying to say is that it probaly has alot of emitter.
Am not an enginnering student so i can't really answer how a microwave is built...
I understand the one on water absorption.It's done using FTIR.Different molecules has different energy requirement needed to bring it to it's excited state.FTIR is base on the principles of atoms excitation.
Let's say we have a compoud and we want to know what molecules are present.
We allow it to undergo FTIR test.A graph will be produce which shows the different peak.
The different peaks would give you the different absorption of wavelength for different compoud.
If water falls in their respective abosprtion of wavelength.then water is present.
So since atom excitation=transmission of atoms to a higher energy level,heat is produced.
However,this is still not the case of mutation.