> discovered microwave energy could be used to heat stuff
It was already well known that EM waves / photons could heat things, sunlight to give the most obvious example.
His discovery was how effectively specific frequency ranges¹ can be at heating food by exciting the water within – effectively enough that the technique once refined would be useful for commercial and even domestic cooking, safely and at practical cost.
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[1] domestic ovens use ±2.45GHz, 915MHz is also effective and used in larger commercial ovens as it can penetrate deeper into the material being heated
2.45 GHz has nothing to do with the physics of water molecules or anything else. You could do the same thing at 13 MHz (diathermy) or 95 GHz (active denial). 2.45 GHz is just where the FCC dumps RF signals they don't want to deal with.
You're correct that other frequencies would work, but have the causality backward when it comes to 2.4-2.5 GHz being a garbage dump. The nascent microwave oven industry was making it into a de facto garbage dump, so the FCC requested that the ITU assign it as an official garbage dump. In other words, the ovens weren't put there because it was a garbage dump, it became a garbage dump because they were already there.
Yeah, good point there. Few licensed services probably wanted to co-exist with the RF leakage from a microwave oven.
I imagine that 2.45 GHz was chosen as a compromise between cavity size/cost and food-penetrating power, but I don't think I've seen a definitive historical answer. The sweet spot for cooking effectiveness might even be at a somewhat-higher frequency, but it would have been more expensive to build ovens at higher frequencies back in the day, and it would also be more expensive to shield them properly. Like the GP suggests, at lower frequencies the size of the whole thing starts to become a problem for household use.
This is pure speculation, but I wonder if it might also have something to do with surplus cavity magnetrons (and the production facilities for them) becoming available post-WWII. I can't find anything definitive on production models, but I was able to determine that the prototype magnetron that Churchill sent to the United States in secret operated at around 3 GHz...pretty close. Maybe cheaper to use the ones that already had production lines set up, if they were otherwise workable?
No, the dimensions of the magnetron changes and the mesh size at the door, otherwise it's just the same.
Penetration depth at higher frequencies is lower, a lower resonant frequency penetrating deeper the tissue while engaging water molecules would be better
At higher frequencies, the problem with 2.4-GHz microwave ovens only tending to cook the outer layers of food becomes even more acute. If you were to actually build a microwave oven at 95 GHz, it would suck because only the very outermost layers of the 'food' would see any meaningful heating.
It was already well known that EM waves / photons could heat things, sunlight to give the most obvious example.
His discovery was how effectively specific frequency ranges¹ can be at heating food by exciting the water within – effectively enough that the technique once refined would be useful for commercial and even domestic cooking, safely and at practical cost.
--
[1] domestic ovens use ±2.45GHz, 915MHz is also effective and used in larger commercial ovens as it can penetrate deeper into the material being heated