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If light is quantized, why are EM spectrum and Blackbody spectrum continuous? 
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#1
Mar1611, 01:58 PM

P: 17

If light is quantized, and is given out in packets, why are the EM wave spectrum and the black body spectrum continuous? I am very confused, can someone offer some explanation? Any input is greatly appreciated.



#2
Mar1611, 02:59 PM

P: 1,521

What is quantized is the energy that can be transferred from the electromagnetic field (= the light) to another system. This energy comes in packets h*f where f is the frequency. This doesn't imply that f is not continuous.



#3
Mar1611, 05:28 PM

P: 661

Transitions between bound states in atoms give an (almost) discrete spectrum, but a continuous spectrum can arise from processes such as Bremsstrahlung, Compton Scattering, boundfree transitions, Synchrotron radiation ...
EM spectrum would be discrete if time is discrete, but it's assumed to be continuous unless compelling evidence to the contrary appears. 


#4
Mar1711, 06:03 AM

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If light is quantized, why are EM spectrum and Blackbody spectrum continuous?



#5
Mar1711, 09:40 AM

P: 661

In thermal radiation from real materials you have molecular radiation which adds all sorts of "random" (continuous) noise, especially in the solid state which may have unbound electrons drifting near the surface. I highlighted some specific processes where an arbitrary continuous variable might be assumed for the photon frequencies, and these will only be discrete if time is discrete in nature, a hypothesis which has no experimental support. 


#6
Mar1711, 11:14 AM

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#7
Mar1711, 11:21 AM

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''quantized'' is just a buzzword meaning that something can be explained by describing it through quantum mechanics, and that _somewhere_ some discrete features show up. Many interesting operators in quantum mechanics have both a discrete and a continuous spectrum. 


#8
Mar1711, 11:23 AM

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#9
Mar1711, 12:37 PM

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Units have nothing to do with discreteness. The spectrum of the hydrogen atom has a discrete part and a continuous part. So energy is both discrete and continuous. According to your argument, since Plancks constant is constant, it follows that time is both discrete and continuous, too. 


#10
Mar1711, 12:47 PM

P: 661

I know you're a very smart guy, so you're obviously having some "overthink" problem here, really, I haven't said anything complicated or controversial. 


#11
Mar1711, 01:01 PM

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#12
Mar1711, 01:09 PM

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#13
Mar1711, 01:11 PM

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#14
Mar2513, 03:38 AM

P: 10

''If light is quantized, and is given out in packets, why are the EM wave spectrum and the black body spectrum continuous? ''
1) Imagine a sine wave, via the Doppler shift we can stretch at and squish it. Therefore the spectrum will appear continuous. 2) There is also the matter of kinetic energy of particles, a single particle can be heated ie from 20 to 30 deg, now it can releases this energy. We don't heat the atom discretely, ie 20.1 20.2 ... therefore the blackbody radiation it will release will trace a continuous spectrum. :) 


#15
Mar2513, 08:26 AM

P: 28

the density of states( number of rasonators EM ssources per unit of space frequency) of black body radiator is a continuous function. However, to obtain the density of states Planck assumed that the distribution was quantised.



#16
Mar2513, 08:31 AM

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Of course, Planck has not derived the BlackBody Spectrum from this wrong assumption but from the right one that any electromagnetic field mode with frequency [itex]\omega[/itex] exchanges energy with the atoms in the walls of the cavity by portions of [itex]\hbar \omega[/itex]. The spectrum itself is continuous (in the limit of cavity extensions large compared to the relevant wavelengths of the radiation, determined solely by the temperature of those walls), i.e., [itex]\omega[/itex] can take any real value.



#17
Mar2513, 10:04 AM

P: 28

So the EM fiels, ω, is continous but the exchange of energy between the field inside of the cavity and the wall of the cavity is discontinous, isn't it? It can only happened in a quantised way



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