# Why do microwave ovens use radiation with such long wavelength?

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:

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:

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 ?

• Might Engineering be better suited for this question? Commented Mar 9, 2015 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." Commented Mar 10, 2015 at 8:03
• @KyleKanos I thought about adding it there, but I decided on Physics instead because my questions is more towards radiation, water molecules etc. Commented Mar 10, 2015 at 8:39
• @gigacyan Hmmm, that would make sense. Can you add it as an answer ? Commented Mar 10, 2015 at 8:39
• 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.. Commented Feb 26, 2017 at 12:49

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.

• Or, for more detail and references, see Electromagnetic absorption by water in Wikipedia. Commented Mar 10, 2015 at 10:54
• Nonsense, microwave frequency was selected for physical reasons (penetration of food) and the frequencies were kept free from communication use. Commented Apr 1, 2015 at 16:11
• @Georg: can you prove your bold statement? Commented Apr 2, 2015 at 7:48
• @gigacyan Microwave heating came before the allocation of the ISM bands in 1947. Indeed in the link you will note a specific request from the US regarding the 2.4GHz band. en.m.wikipedia.org/wiki/ISM_band#History Commented Apr 4, 2017 at 7:39

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.

• 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. Commented Mar 10, 2015 at 8:00
• Water resonates at 22 GHz more or less. 2.45 GHz and 915 GHz are just frequencies agreed on by countries internationally. Commented May 19, 2016 at 5:55