By Bhavesh


2015-03-28 10:43:24 8 Comments

We know that any object above absolute zero emits electromagnetic radiation. So hotter the object shorter the wavelengths. In the electromagnetic radiation spectrum radio waves has the longest then microwave, then infrared, then visible light, then ultraviolet, then x-ray, then gamma rays. Why do we use microwaves in microwave oven when infrared and visible light are much hotter and how do microwaves cook food when they are cooler than visible light and others.

4 comments

@John Rennie 2015-03-28 11:26:26

In microwave ovens what matters is how much energy the radiation carries and how that energy is absorbed by the food. Visible light and IR are rapidly absorbed by most foods, so they would only heat the outer layer of the food. You'd get food with the outside carbonised and the inside raw.

Microwaves are far less strongly absorbed by foods, so they penetrate deep into the food and can heat the interior. Even so large objects won't be heated throughout, which is one reason why microwave cooking instructions frequently advise a multi stage process of heating, letting the food stand then heating a final time.

Microwave ovens often include IR heating as well as the microwave heating. This is done so you get food with a browned exterior and heated throughout.

The answers to Why do microwave ovens use radiation with such long wavelength? give a nice discussion of why the exact wavelength used was chosen. The frequencies commonly used in microwave ovens are 2.45 GHz (12 cm) for home ovens and 915 MHz (38 cm) for industrial overs. Much higher frequencies are not used due to the cost of the magnetron, while much lower frequencies would not work because the wavelengths would be too big to allow a half wavelength to fit in the oven.

Finally, you say:

Why do we use microwaves in microwave oven when infrared and visible light are much hotter and how do microwaves cook food when they are cooler than visible light and others.

But this is a slight misunderstanding. The wavelength of light emitted is indeed related to the temperature of the source, but light itself doesn't really have a temperature in the sense that matter does. Light transfers energy, and if this energy is absorbed it will heat the food. However the amount of heating is just related to the intensity of the EM radiation and the abosrption cross section. The wavelength makes a difference only insofar as it affects the absorption cross section.

@Bhavesh 2015-03-28 16:34:40

if this is the principle behind microwaves in cooking food then why can't we use radio waves instead, they will go deeper inside large foods?

@Lightness Races in Orbit 2015-03-28 16:41:22

@Bhavesh: What prevented you from performing basic research? Wikipedia says "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", and this was [..]

@Lightness Races in Orbit 2015-03-28 16:42:36

[..] quoted on the Q&A John linked you to, along with: "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". It seems you didn't bother to read it. :(

@John Rennie 2015-03-28 17:30:29

@LightnessRacesinOrbit: I think your criticism is a bit unfair. Radio waves are indeed used for heating in induction furnaces. Well, the heating is magnetic not electrical, but frequencies up to 400kHz are used and that's into the radio range. Induction furnaces are extremely effective at heating food, but they tend to produce a lot of electrical interference.

@Lightness Races in Orbit 2015-03-28 17:31:08

@JohnRennie: Do you disagree that the OP is consistently showing no research effort?

@John Rennie 2015-03-28 17:35:57

@Bhavesh: it's mainly that it's easy to produce high energy microwaves efficiently in a magnetron. The magnetron and oven are designed to be an integral number of wavelengths in size so the oven contains a standing wave. Radio waves are generally too large for this - FM wavelengths are a few metres - so it would be hard to make a radio oven powerful and efficient enough to be useful. It would be better for even heating though.

@John Rennie 2015-03-28 17:36:47

@LightnessRacesinOrbit: what seems obvious to us god-like nerds is not necessarily obvious to normals.

@Lightness Races in Orbit 2015-03-28 17:51:40

@JohnRennie: I never said it was obvious. I said the OP is failing to perform any research, or to read the material presented to him.

@John Rennie 2015-03-28 18:05:47

I think it's good to err on the side of being nice

@Bhavesh 2015-03-29 15:04:20

@JohnRennie thank you very much. I failed to perform a good research because i find physics.stackexchange.com the best place to ask questions and i get very fast replies.

@jiggunjer 2015-04-08 20:19:13

This answer contains a mistake. Based on the absorption spectrum of water, microwaves are absorbed BETTER than IR or VIS light. So apart from being more expensive to generate, higher frequencies are less efficient.

@Ben Crowell 2019-01-12 22:48:36

@jiggunjer: Based on the absorption spectrum of water, microwaves are absorbed BETTER than IR or VIS light. Not true. A chicken breast is 100% opaque to visible light, whereas it is partially transparent to microwaves.

@jiggunjer 2019-01-22 06:08:14

@BenCrowell A chicken breast is 100% opaque to visible light, whereas it is partially transparent to microwaves. Two statements lacking evidence. You can easily google the absorption coefficient for water as a function of wavelength.

@my2cts 2019-01-09 10:13:34

Microwaves are very efficiently absorbed by water molecules in the food. Water molecules are small electrical dipoles that wiggle at microwave (GHz) frequencies when in the liquid state. Microwaves resonate with this wiggling motion. Watermolecules are everywhere in our food and drinks so via these the rest of the food constituents are heated by conduction. In ice the molecules wiggle at kHz frequencies which is why defrosting in a microwave oven really does not work.

@Luaan 2019-01-09 09:47:53

Both visible light and IR is used for cooking. Visible light is used in solar ovens (basically mirrors concentrating sunlight on a food item). IR light is used in a normal oven (the walls of the oven are heated, which causes them to radiate significant amount of heat as IR); grills use the same approach, just with higher temperatures. If you've ever baked/grilled something wrapped in tinfoil, the point of that is to exclude the IR radiation (and rely on air heating instead).

So the first part of the answer to your question is simple enough - if microwave ovens used visible or IR light, we would not call them microwave ovens, just ovens.

But microwave ovens weren't sold as "this is a lot more awesome, because it uses microwaves!". The selling point of microwave ovens is that they heat the food with a lower power input. This requires two things; first is conversion efficiency: how much electricity it takes to produce the radiation. However, a simple oven (gas or electric) is hard to beat in that respect - it's very close to 100% efficient, since the waste heat is actually what causes the heating. Microwave ovens are built to use radiation sources that are as efficient as possible (while also not interfering with other electrical devices), but those are still less efficient than a piece of resistive wire or an ordinary light bulb.

So if microwave ovens aren't more efficient in power conversion, how come they're so much faster at heating food? Their selling point (besides things like small size and weight) was that they heat the food, not the container, and the oven, and the air, and.... This doesn't sound all that important, but it's actually a very big deal. Ovens need a lot of energy to heat up; not a big deal if you're cooking something for eight hours, or if you make a lot of meals (e.g. pizza ovens in a restaurant), but if you just want to heat a quick meal, most of the energy is wasted. Stoves need to heat the pan, and of course, even when you're just reheating a meal, you often need to heat quite a bit of extra water (and vaporize most of it, so the food isn't soggy).

Are microwaves uniquely suited for this task? Kind of. They're not readily absorbed by kitchenware, and they're not even scattered all that much (compared to e.g. visible light reflecting off a white plate). At the same time, they're extremely well absorbed by both water and fats, and most of our foods are full of water. So you definitely want very long wave radiation. How long? That depends on other constraints. Too long waves mean the oven becomes too large, and one of the big benefits of microwave ovens is that they're very small appliances. Many frequency bands of long-wave radiation are already used for communication - radios, Wi-fi, cellphones... so you don't want to interfere with that. You also need to avoid inducing current in wires outside of the microwave. Short waves are a lot more tricky to produce using the equipment - a radio is more power efficient than a microwave, and visible light is even worse. An actual engineer who deals with microwave ovens could probably tell you a hundred constraints off the top of his head :)

@Thaina 2015-03-28 16:24:33

As @john-rennie have state out. Shorter wavelength tend to absorb and heat the outer layer of object. Microwave using long wavelength to penetrate to inside and also it use the property of dipole moment of water molecule to heat things, not directly heat by let object absorb energy

If we just use short wave then you can only get burnt outside and raw inside

@thermomagnetic condensed boson 2019-06-15 21:00:15

So you're saying that X-rays would not penetrate food deeply at all? Wow... Then how come X-rays can go through our bodies that easily?

@Thaina 2019-06-16 05:18:59

@thermomagneticcondensedboson It's completely opposite. It's because your skin and bone will absorb XRay that's the reason why we can use XRay to image your internal organ. And that's also the reason why it require so much energy and protection just to flash XRay pulse. To fire the XRay wave so much that it pass through you is very high energy. Most of it got absorbed by your body

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