To Big Boomer
Beware of talking down the heating effects of different radio frequencies. Domestic microwave ovens operate at 2.45 GHz for two main reasons:
a) The size of the resonant cavity and the cost-efficiency of producing the magnetron for that frequency are both close to ideal at 2.45 GHz.
b) The actual penetration depth at that frequency is in the cm range, the actual value depending on the salt content of the water but, again, ideal for food heating. The first resonant peak for the water molecule itself is above 1 THz and the highest peak is in the infrared range. Microwave ovens cook quickly because the penetration (to whatever depth) is immediate rather than the gradual with conventional heating. In practice though, a combination is used (the instruction to "stand for x minutes before serving" provides extra insurance that the heating-through process is as complete as it can be).
also, the frequency band around 2.45 GHz was one of the first pieces of radio spectrum globally assigned to ISM - Industrial, Scientific and Medical usage - well before there were such things as microwave ovens.
Actually, anywhere in the 900 MHz to 5 GHz range is technically fine for microwave cooking and some professional ovens (big ones) operate at 915 MHz. Remember also, that the medical treatment known as Diathermy (tissue heating) operates way down at 27 MHz (did someone mention CB radio?) while frequency-hopping Bluetooth devices are another user of the same spectrum.
The word "microwave" means nothing special other than "tiny wave". Depending on which text book you read, the microwave range starts as low as 300 MHz (the VHF/UHF boundary) or 1000 MHz (a nice round number). They're just radio waves which are the lowest energy part of the electromagnetic spectrum which goes on to include infrared, visible light, ultraviolet, x-rays and gamma rays.
Ron Schmitt's "Electromagnetics Explained" (pub: Newnes) is a good primer for those wishing to understand the phenomenon a bit and be able to debunk the quacks with their patches and crystals.