UV catastrophy and PLANCK - Urgent (1 Viewer)

[Mr_Kap]

Ok. Can somebody please explain to me about black bodies, and why classical physics thought there would be infinite intensity for smaller wavelengths? And is there a wavelength where intensity is zero, or is the Intensity = 0 an assymptote...and why?

Also, from the student's guide to HSC physics it says:

"Planck resolved this problem with his hypothesis of quantised radiation which explained the experimental data, stating that radiation could only occur in small packets which he called “quanta”. The energy contained within a single quanta is dependant only on the frequency of the radiation according to the formula E = hf. Further, the vibration states of atoms in the black body cavity were also quantised, meaning that they could only have specific discrete frequency values. Since energy is only emitted when these atoms change vibrational states moving to a less energetic state, the energy released is also quantised."

I don't get how Planck saying that EMR occuring in quanta and not waves explains the obserations observed? Why would the light as packets of energy make any difference?

 

[InteGrand]

Ok. Can somebody please explain to me about black bodies, and why classical physics thought there would be infinite intensity for smaller wavelengths? And is there a wavelength where intensity is zero, or is the Intensity = 0 an assymptote...and why?

Also, from the student's guide to HSC physics it says:

"Planck resolved this problem with his hypothesis of quantised radiation which explained the experimental data, stating that radiation could only occur in small packets which he called “quanta”. The energy contained within a single quanta is dependant only on the frequency of the radiation according to the formula E = hf. Further, the vibration states of atoms in the black body cavity were also quantised, meaning that they could only have specific discrete frequency values. Since energy is only emitted when these atoms change vibrational states moving to a less energetic state, the energy released is also quantised."

I don't get how Planck saying that EMR occuring in quanta and not waves explains the obserations observed? Why would the light as packets of energy make any difference?

Read Section 10.5 starting on Page 5 of that document for info about the UV catastrophe. Basically, classical physics came up with the Rayleigh-Jeans Law for the spectrum of black-body radiation as a function of frequency :

, where k is the Boltzmann constant, T is the temperature of the black body, and is the frequency of light in question. The area under this curve () should represent the total energy emitted at the given temperature T. Unfortunately, with this model, the total energy emitted would be , and infinite energy is a problem. So a new model for , and hence a new model of physics, was needed.

As that document will explain, Planck's assumption of discretised energy states for black-body oscillators resulted in finite energy emission.

Something like this is needed for HSC Physics: At first, Planck was just doing this thinking it was a mathematical trick to get a finite answer. But Einstein believed that his assumption of discrete energy was actually true, and came up with his Photoelectric effect based on the idea of discrete packets of energy in photons, etc.

And check this thread: http://community.boredofstudies.org/265/ideas-implementation/336155/uv-catastrophe.html

 

[InteGrand]

I don't get how Planck saying that EMR occuring in quanta and not waves explains the obserations observed? Why would the light as packets of energy make any difference?

The explanation is not simple and is very mathematical (like most things in real physics).

I found this PDF to be very helpful for explaining these things, but it assumes some maths: http://disciplinas.stoa.usp.br/pluginfile.php/48089/course/section/16461/qsp_chapter10-plank.pdf

 

[Mr_Kap]

The explanation is not simple and is very mathematical (like most things in real physics).

I found this PDF to be very helpful for explaining these things, but it assumes some maths: http://disciplinas.stoa.usp.br/pluginfile.php/48089/course/section/16461/qsp_chapter10-plank.pdf

Ahh ok.
Can you just clarify why classical physics thought the curve would be increasing for smaller wavelengths? And what actually IS a black body?

 

[InteGrand]

Ahh ok.
Can you just clarify why classical physics thought the curve would be increasing for smaller wavelengths? And what actually IS a black body?

This was because of the Rayleigh-Jeans Law, a consequence of the laws of classical physics (which did NOT assume that EMR was emitted in discrete packets): http://en.wikipedia.org/wiki/Rayleigh–Jeans_law (see the first equation for the equation of the curve as a function of wavelength).

As that equation (i.e. Rayleigh-Jeans law) shows, as , the spectral radiance tends to infinity.

As this is not possible in reality, it meant something was wrong with the assumptions of classical physics when applied to blackbody radiation (because the Rayleigh-Jeans law was a consequence of these assumptions, as it was derived from them, so if classical physics was true for blackbody radiation, we should have observed Rayleigh-Jeans law in practice, but we don't (for small wavelengths)).

Here's the derivation of the Rayleigh-Jeans law: http://www.applet-magic.com/rayleighjeans.htm

And a black body is defined here: http://en.wikipedia.org/wiki/Black_body

 

[Mr_Kap]

thx.

last questions....is there a maximum wavelength of light?
And what about x-rays and gamma rays...why does the graph start with wavelengths of uv?

and is temperature the same as intensity?

 

[Mr_Kap]

Super ultra bump

 

[InteGrand]

thx.

last questions....is there a maximum wavelength of light?
And what about x-rays and gamma rays...why does the graph start with wavelengths of uv?

and is temperature the same as intensity?

Wavelengths essentially can go very large (though not infinite). See http://en.wikipedia.org/wiki/Electromagnetic_spectrum

As that shows, there are wavelengths that are of the order of Mm (mega metres, or millions of metres).

The upper limit for the wavelengths is the size of universe (as that page says).

The graph talks about UV wavelengths mainly because it is in this range of wavelengths and shorter that the problem becomes apparent (for longer wavelengths, the Rayleigh-Jeans law is a good approximation to observed results, but in this range, the Rayleigh-Jeans law quickly tends to infinity, thus being problematic as this is not observed in reality).

And temperature is a different concept to intensity (http://en.wikipedia.org/wiki/Temperature). However, you can see that the spectral radiance (i.e. the curve, similar to intensity, as http://en.wikipedia.org/wiki/Radiance says that 'Historically, radiance is called "intensity" and spectral radiance is called "specific intensity". Many fields still use this nomenclature. It is especially dominant in heat transfer, astrophysics and astronomy.') is affected by temperature, if you look at the equations for the curves (both evident in Rayleigh Jeans law and Planck's law – the capital T in these equations will signify temperature, so you can see how the curves are affected by temperature).

 

[Mr_Kap]

alright thx a bunch!