- #1
fluidistic
Gold Member
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First of all I'm not sure this thread should be in this section or in the Thermodynamics one.
I don't understand something. If I'm not wrong, the wavelength of photons used in microwave oven cooking is around [tex]122 mm \sim 10^{-1} m[/tex], much greater than visible light photons (450 to 750 nanometers, from my memory, which is about [tex]5x10^{-6}m[/tex]. So the frequencies of micro wave is much lesser than the one of visible light. Since [tex]E_{\text{photons}}=h \nu[/tex] where [tex]\nu[/tex] is the frequency, it means that photons of visible light are much energetic than the microwave's ones, in fact by a factor of [tex]10^5[/tex]. I've heard that food (especially the ones containing water) can be cooked by microwaves because the wavelength of the waves "matches" the water molecules or so (though I don't understand this since it's they are at a totally different scale) and they enter in resonance so that in fact water molecules absorbs well the microwave. So far so good.
Now comes my problem. If I paint anybody of black so that it absorbs say around 70% of visible light, how come it doesn't heat much more than water in a microwave? Photons of visible light are so much more energetic and are mostly absorbed by the material (by its surface at least) that I can't understand why the painted "black" body doesn't heat up more than water in a microwave.
I know Stefan-Boltzmann's law of radiation so I can understand that a black body wouldn't heat that much because it also emits part of what it absorbs... but what happens in the microwave, I have no idea.
I don't understand something. If I'm not wrong, the wavelength of photons used in microwave oven cooking is around [tex]122 mm \sim 10^{-1} m[/tex], much greater than visible light photons (450 to 750 nanometers, from my memory, which is about [tex]5x10^{-6}m[/tex]. So the frequencies of micro wave is much lesser than the one of visible light. Since [tex]E_{\text{photons}}=h \nu[/tex] where [tex]\nu[/tex] is the frequency, it means that photons of visible light are much energetic than the microwave's ones, in fact by a factor of [tex]10^5[/tex]. I've heard that food (especially the ones containing water) can be cooked by microwaves because the wavelength of the waves "matches" the water molecules or so (though I don't understand this since it's they are at a totally different scale) and they enter in resonance so that in fact water molecules absorbs well the microwave. So far so good.
Now comes my problem. If I paint anybody of black so that it absorbs say around 70% of visible light, how come it doesn't heat much more than water in a microwave? Photons of visible light are so much more energetic and are mostly absorbed by the material (by its surface at least) that I can't understand why the painted "black" body doesn't heat up more than water in a microwave.
I know Stefan-Boltzmann's law of radiation so I can understand that a black body wouldn't heat that much because it also emits part of what it absorbs... but what happens in the microwave, I have no idea.