why microwaves heat food and not visible light?

[lucio_]

TITLE EDIT:

why microwaves heat food and visible light doesn't?

 

 

ok, here's my first question.

 

in the (recent) past I've read a book of introductory chemistry, and one of introductory astronomy, but I have many practicaly questions left, and here is the first:

 

so, the shorter the wavelength the more the power the photons carry, right?

 

so if microwaves are longer than, say, visible light, why when I put my meal in the microwave oven it gets hot and it doesnt get how while leaving in the kitchen or elsewhere with the visible light?

maybe because they push on the oven many many beams? so then why don't use visible light wavelength or infrared or some other shorter wavelength?

 

 

thaaaank you!

Edited February 2, 2010 by lucio_

[farmboy]

In any wave you can calculate the frequency and wavelength quite easily, using velocity=frequency x wavelength. Since the velocity of light is constant, that means that as the wavelength increases the frequency decreases. The frequency is directly related to the energy by the formula frequency=energy/plancks constant, so yes it is true that at lower wavelengths the frequency of the wave is lower and the energy is higher.

 

I don't think thats particularly important in this case though. The important part (I'm guessing) is that the frequency of microwaves is somewhere in the region of the natural vibrational frequency of water, which causes it to vibrate massively where visible light wouldn't. This vibration increases the temp of the whole food since food is largely water.

[insane_alien]

well, most ovens use IR radiation for heating food, shorter wavelength than microwaves and visible light CAN heat stuff, you just need to shove enough of it onto the thing you want to heat up.

 

a microwave is what, 1kW on an area maybe 100cm^2 light is 1kW on 10000cm^2 not counting reflection.

 

microwaves fire more energy into it than natural lighting does. you could use it but why bother,.

[Cap'n Refsmmat]

You could use natural light, but it would have to be incredibly bright. Ever used a magnifying glass and the Sun to set something on fire? It can be done.

[Klaynos]

I don't think thats particularly important in this case though. The important part (I'm guessing) is that the frequency of microwaves is somewhere in the region of the natural vibrational frequency of water, which causes it to vibrate massively where visible light wouldn't. This vibration increases the temp of the whole food since food is largely water.

 

Is the key to understanding this. The microwaves are absorbed very strongly by water molecules, these then non-radiatively decay which results in the food heating up as a whole.

[swansont]

Is the key to understanding this. The microwaves are absorbed very strongly by water molecules, these then non-radiatively decay which results in the food heating up as a whole.

 

Plus, they tend to penetrate of order a wavelength for a reasonably strong absorber. For visible light, that's less than a micron, while for microwaves, it's centimeters. So the energy that's deposited by microwaves is more spread out and more penetrating. The energy deposited by visible light would all be at the surface, and you rely on conduction to heat further in.

[CaptainPanic]

Is the key to understanding this. The microwaves are absorbed very strongly by water molecules, these then non-radiatively decay which results in the food heating up as a whole.

 

Plus, they tend to penetrate of order a wavelength for a reasonably strong absorber. For visible light, that's less than a micron, while for microwaves, it's centimeters. So the energy that's deposited by microwaves is more spread out and more penetrating. The energy deposited by visible light would all be at the surface, and you rely on conduction to heat further in.

 

I think these two posts together contain the answer.

 

Microwaves can penetrate the food much better than visible light or infrared. Microwaves (sort of) heat the food equally through all the food, while an oven heats from the outside, forming a nice crust on top of the food while the food can still be cold on the inside.

 

A bonus is that microwaves are absorbed more strongly by water than other materials. The absorption isn't all that great, otherwise the waves would not penetrate so deeply into the food... but it heats the water more than for example the container in which the water is placed.

 

The penetration of the waves into the food, and the resulting ability to heat the food in a uniform manner means that the food can be heated much faster... and that's why microwaves became the machines they are.

 

But you can definitely use normal light to heat something. Place something in the sun during summer, and it will heat up.

If something is in the light, but not heating up, it means that it cools itself just as fast as it heats by the absorbed visible light. In other words: the apples in your kitchen absorb visible light and all other kinds of radiation, but emit (a little) infrared light.

 

And an oven uses infrared (and partially also visible light if some parts glow red)... unless you have moving air in the oven - then you have a completely different heat transfer which is called "convection" (moving air) - but that has little to do with radiation and is a different chapter altogether.

[Externet]

TITLE EDIT:

why microwaves heat food and visible light doesn't? QUOTE]

 

---> http://www.lge.com/au/kitchen-appliances/lightwave-ovens/index.jsp

 

---> http://en.wikipedia.org/wiki/Solar_cooker

[Baby Astronaut]

Is the key to understanding this. The microwaves are absorbed very strongly by water molecules, these then non-radiatively decay which results in the food heating up

What does non-radiative decay mean? I looked it up in several places, even Google Images, and couldn't grasp it whatsoever.

 

But you can definitely use normal light to heat something. Place something in the sun during summer, and it will heat up.

I thought it'd get heated by particles from the sun, rather than light itself.

[swansont]

What does non-radiative decay mean? I looked it up in several places, even Google Images, and couldn't grasp it whatsoever.

 

Relaxation via means other than emitting a photon.

 

I thought it'd get heated by particles from the sun, rather than light itself.

 

What particles are you thinking of?

[Baby Astronaut]

Relaxation via means other than emitting a photon.

Would that include nuclear decay since it's not a photon that's being emitted or radiated?

 

What particles are you thinking of?

Solar wind. I had thought its particles collided with air, forming a long chain of reactions where eventually air particles collide with the ground. But I just learned the solar wind mostly avoids us.

 

So now I'm confused again. I thought heat's the internal kinetic energy of a bunch of particles in a system. So in order to transfer the internal kinetic energy from one thing to another, wouldn't you need particles? Unless light itself contains heat, which if that's the case it doesn't really make sense.

[Sisyphus]

Ok, terms: heat is a transfer of energy. Things do not "have" heat. They have energy. The measure of average kinetic energy of particles is temperature.

 

Transferring energy in the way you describe, by "knocking particles together" until the average energy of the two bodies evens out, is called conduction. However, photons (light) also have energy, which they transfer to whatever they hit. This is called radiation. It is the sun's radiation, its light, that keeps all of us alive.

[Baby Astronaut]

Ok, terms: heat is a transfer of energy. Things do not "have" heat. They have energy. The measure of average kinetic energy of particles is temperature.

 

Transferring energy in the way you describe, by "knocking particles together" until the average energy of the two bodies evens out, is called conduction. However, photons (light) also have energy, which they transfer to whatever they hit. This is called radiation. It is the sun's radiation, its light, that keeps all of us alive.

Yeah temperature's what I meant.

 

So light has temperature. It's a really odd thought, because photons would have an inner kinetic motion of c, and therefore light should measure to have a temperature of c.

[npts2020]

Yeah temperature's what I meant.

 

So light has temperature. It's a really odd thought, because photons would have an inner kinetic motion of c, and therefore light should measure to have a temperature of c.

 

Light does not have temperature, it has energy. Only substances with atoms have temperature. Light can impart some or all of its energy to a substance to increase the temperature of the substance. (c is not temperature)

[swansont]

Would that include nuclear decay since it's not a photon that's being emitted or radiated?

 

 

Most examples of decay are not from an excited state, so no. But radiation includes any energetic particle that's emitted.

[Baby Astronaut]

Light can impart some or all of its energy to a substance to increase the temperature of the substance.

But what other energy besides kinetic can light have? If light transfers any kinetic energy, it'd no longer travel at c.

 

Most examples of decay are not from an excited state, so no.

Are most of those examples particle decay?

 

But radiation includes any energetic particle that's emitted.

For instance, light? Or would it not be considered to be a particle and/nor energetic?

[jdurg]

But what other energy besides kinetic can light have? If light transfers any kinetic energy, it'd no longer travel at c.

 

 

Are most of those examples particle decay?

 

 

For instance, light? Or would it not be considered to be a particle and/nor energetic?

 

Light always travels at c. The frequency of a lightwave can be calculated using the equation c = frequency x wavelength. The wavelength of a lightwave can be calculated by the equation E (energy) = h(Planck's Constant) x v (frequency).

 

So because Planck's constant and the speed of light are set constants, when light loses energy (E gets lower) the frequency of the light would have to decrease. (If the total energy of the light was 20, and Planck's constant was set at 2, then the frequency would be equal to 10. If the total energy dropped down to 10, then the frequency would drop to 5 in order to keep things equal).

 

So if the frequency drops, then the wavelength would have to increase. As a result, the light would go from violet to red if the transition was in the visible range. When light loses energy, the wavelength of the light will increase. Theoretically, one could take UV Radiation and pass it through a medium which would absorb energy from it and make it visible.

[Baby Astronaut]

Holy crap. That's a very good explanation.

 

So, basic question: higher frequency means shorter wavelengths, and in effect it should also mean a greater number of photons. Is that correct? If so, when you lower the frequency of light, does that mean photons vanish if their total number is reduced?

[swansont]

Theoretically, one could take UV Radiation and pass it through a medium which would absorb energy from it and make it visible.

 

I would call such a mystical device a fluorescent light bulb.

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Holy crap. That's a very good explanation.

 

So, basic question: higher frequency means shorter wavelengths, and in effect it should also mean a greater number of photons. Is that correct? If so, when you lower the frequency of light, does that mean photons vanish if their total number is reduced?

 

It depends on the process. Photon number is not a conserved quantity — you can make and destroy them. Some processes make photons, some absorb them. One process can turn one photon into two, and another does the opposite.

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But what other energy besides kinetic can light have? If light transfers any kinetic energy, it'd no longer travel at c.

 

It can scatter and give up some energy. But yes, a photon's energy is all kinetic, in a sense.

 

 

Are most of those examples particle decay?

 

 

For instance, light? Or would it not be considered to be a particle and/nor energetic?

 

Light is considered a particle in this analysis. Alpha decay and beta decay can be accompanied by photons. All of the emitted particles are considered radiation. You can also have an excited nucleus emit a photon (or sometimes a neutron, under the right conditions) when it drops to a lower state.

[lucio_]

thanks everyone for a very interesting read.

 

still I haven't grasped everything though, and maybe because a new subject unknown to me came up, what do you mean by:

"natural vibrational frequency of water" ?

Edited February 13, 2010 by lucio_

[insane_alien]

the atoms in a molecule are jiggling about. if you fire photons at it that match this frequency of jiggling then they are more likely to be absorbed.

 

it is important to note that this has nothing at all to do with new agers and crystals and all that rubbish.

[lucio_]

the atoms in a molecule are jiggling about. if you fire photons at it that match this frequency of jiggling then they are more likely to be absorbed.

 

it is important to note that this has nothing at all to do with new agers and crystals and all that rubbish.

 

ok, and this jiggling of the atoms has nothing to do with the temperature? is it a constant, and different in each different atom?

[farmboy]

thanks everyone for a very interesting read.

 

still I haven't grasped everything though, and maybe because a new subject unknown to me came up, what do you mean by:

"natural vibrational frequency of water" ?

 

Reading back, I think I was wrong to mention the natural vibrational frequency, wasn't quite sober at the time. I think perhaps that has something more to do with the vibration of solid lattices. Basically the frequency that the lattice will naturally vibrate at, and so by matching that frequency you can get a sort of chain reaction where the frequency and velocity of the standing wave (sorry not sure about terminology don't know a lot of physics) increases rapidly keeping the wavelength the same, meaning the total energy of the vibrations increases massively. This is how opera singers can smash glasses I think.

 

I suppose the water example is like the quantum analog of this. I'm thinking now that with the water the important part is the fact that the water molecule is best able to absorb photons with energy levels that fall within the microwave band. Once the molecule is excited like this it will want to lose energy again, and can do so in a number of ways (I think this was mentioned previously). One of these ways would be to increase the frequency of vibration, which would inturn heat the food up.

[swansont]

ok, and this jiggling of the atoms has nothing to do with the temperature? is it a constant, and different in each different atom?

 

The jiggling has everything to do with temperature — temperature is related to the average kinetic energy of the ensemble of molecules. The more they jiggle, the hotter they are.

 

The frequency range that a material will absorb varies with the material.