IIRC, the precise frequency was further narrowed to _not_ be absorbed as well by water, because then all the energy would hit the surface of your food and not t

IIRC, the precise frequency was further narrowed to _not_ be absorbed as well by water, because then all the energy would hit the surface of your food and not the inside (although I'm still not sure if that is relevant at a wavelength that is almost as large as the food you'd want to heat). And then, again IIRC, finally pinned down slightly to the side of that because of the parts that could be sourced to build the first ovens.