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According to Wikipedia:

Consumer ovens usually use 2.45 gigahertz (GHz)—a wavelength of 12.2 centimetres (4.80 in).

Typically, I put the dish inside the oven in its center. I suspect most people do the same:

typical dish placement

Now, because the plate is in the center, it will remain more or less stationary as the oven's dish rotates.

In my case, this causes only part of the food to be heated, only certain areas. Other parts remain cold.

What I've done to solve this is not place the dish in the center, but on the edge:

away from center haha

This fixes the problem and my food is more uniformly heated.

I suspect that this is because of the long wavelength which, combined with a stationary food plate, make certain areas of the plate unreachable.

This makes me wonder: Is there a specific reason why oven manufacturers don't switch to a shorter wavelength ? (let's say 4cm) Are there any essential physical properties of the water or fat molecules that would prevent shortening the wavelength ?

Radu Murzea
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  • Might Engineering be better suited for this question? – Kyle Kanos Mar 09 '15 at 19:17
  • If you read the Wikipedia article further, you'll find that "The microwave frequencies used in microwave ovens are chosen based on regulatory and cost constraints." and "[higher frequencies] are not used for microwave cooking because of the very high cost of power generation at these frequencies." – gigacyan Mar 10 '15 at 08:03
  • @KyleKanos I thought about adding it there, but I decided on Physics instead because my questions is more towards radiation, water molecules etc. – Radu Murzea Mar 10 '15 at 08:39
  • @gigacyan Hmmm, that would make sense. Can you add it as an answer ? – Radu Murzea Mar 10 '15 at 08:39
  • I think food doesnt heat properly when you keep it in the centre because various EM waves are not hitting it after reflection, but when you keep it in other positions, different waves coming at different angles are able to hit it, so i am just speculating.. – Sarthak Sharma Feb 26 '17 at 12:49
  • While browsing through the links here, I found this visualization of distribution of microwave power: https://www.evilmadscientist.com/2011/microwave-oven-diagnostics-with-indian-snack-food/ – user27542 Jan 09 '19 at 09:28

2 Answers2

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I wonder if a bunch of microwave questions that appeared here recently is related to an enlightening (as usual) recent what if by xkcd.

The radiation used in microwave ovens is not resonant for water molecules and industrial microwave ovens use 915 MHz (probably, because larger cavities can produce more power). The frequency of 2.45 GHz is chosen because it falls in one of the bands not reserved for communication purposes. According to Wikipedia, the next available band would be at 5.8 GHz. A powerful magnetron working at this frequency is feasible but way too expensive for a household appliance. If such magnetrons ever get cheaper, it is likely that there will be a luxury microwave marketed for its uniform heating profile.

gigacyan
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The wavelengths that microwaves use allows the waves to resonate with the water to heat it up, with the added value that they are easily blocked to prevent damage to objects outside of the machine.

Misc.nerdiness
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    This is a common misconception. Water (and any other polar molecule) absorbs the whole range of microwave frequencies. It is not resonant with any molecular motion, the molecules just jiggle around, heating the environment. From Wikipedia: industrial ovens use 915 MHz. – gigacyan Mar 10 '15 at 08:00
  • Water resonates at 22 GHz more or less. 2.45 GHz and 915 GHz are just frequencies agreed on by countries internationally. – Old_Fossil May 19 '16 at 05:55