Something that I’ve noticed across most of the microwave ovens that I’ve used is that when they hum while cooking food, I can pick out 2 distinct tones. One of them is pretty clearly 60 120 hz, the 2nd harmonic of the AC power frequency. The other is consistently a minor 7th above that (which would be somewhere around 106-108 212-214 hz depending on the exact ratio). What causes this 2nd frequency to be produced?
Edit: after checking against a tone generator, the low frequency is actually 120 hz, double the grid frequency. The question is still the same, just an octave higher.
Yes, microwaves are a poor substitute for an oven but they work fine for vegetables that you might otherwise use a steamer to cook. Stuff like broccoli, beans, carrot pieces etc. Corn on the cob works well too, just give it a few minutes in the microwave with the husk still on.
Anything that’s ok mushy, really. You can also use it to start cooking something that you finish somewhere else.