Black body radiation (1 Viewer)

[flaminwaffle]

Could somebody help me with this dot point:
[FONT=&quot]Identify Planck’s hypothesis that radiation emitted and absorbed by the walls of a black body cavity is quantised.[/FONT]

I kinda understand the black body radiation stuff, but i have no idea what that has to do with the quantum theory/planck's hypothesis. i'd like it if somebody could explain this from basics, if possible.

ive been reading up on this for hours and i still don't understand it :mad1:

thanks a lot

 

[study-freak]

LOL Black body radiation is exactly why Planck had to hypothesise about quantisation of energy!
Hence they r related
Don't have time to write explanation now but 'll do if I have time later

Edit: See annabackwards' post below
Although there are some errors there, they are minor and won't affect your understanding. (Just that they are 'errors' because they are not in perfectly accurate wording.)
Many HSC markers wouldn't even pick it up, IMO.

 

[annabackwards]

Planck's quantum theory has a lot to do with black body radiation as it solved the "UV catastrophe".

Classical theory implied that as wavelength decreased (and frequency increased), the black body would emit radition of intensity that would continually increase ie eventually reach infinity.

But experimental results did not reflect the classical theory at all and so the problem became known as the "UV catastrophe" (look at the graph and you'll see why - the curve for classical theory reaches infinity at the UV wavelength band).

Planck's quantum theory basically reduced the radiation intensity the black body would be able to emit and so solved the theory - you should know the rest

 

[Bernie Mac]

Just to sorta add something on to annabackward's explanation, you could also mention how the curves of Planck's hypothesis all peak at some point therefore is a maximum curve...this is important and is what distinguishes between quantum and classical theories for black body radiation

 

[raniaaa]

the 'classical' wave theory of light predicted that as the wavelength of radiation emitted (from a black body) becomes shorter, the radiation intensity would increase. as the energy decreased in wavelength, the intensity of the radiation emitted fromthe hole in the black body approach infinity.

this increase in energy would violate the principle of conservation of energy and was called the 'ultraviolet catasrophe'.

however, experimental data from black body experiments showed the radiation intensity curve corresponding to a givern termperature has a definite peak, passing through a maximum then declining.

in 1900, planck assumed that the radiant energyk, although exchanged between the particles of the black body and the radiant energy field in continuous amounts, could be treated as if it were exchanged in 'lumps'. each average packet, a 'quantum of energy', could be be described by e= hf

later on einstein explained planck's work in the following way:
"the energy associated with the radiation from a black body is concentrated in packets of energy called photons. a photon is the smallest amount of radiation enegy possible at a particular frequency. a photon cannot transfer pat of it energy: it can only transfer all of its nergy or none of it."





hope that helps

 

[JasonNg1025]

The temperature in the black body (which determined the energy of the radiation coming out) would peak at a point where the energy coming out corresponded to the particular photon energy, hence a peak at a particular frequency in each temperature.

At higher frequencies, the energy in a photon is higher, so there's less probability that the energy coming out of the black body would fill in those high energy photons, so the curve comes back down. With higher temperatures, it's more likely for the black body to fill in higher energy photons.

EDIT: arr not too sure about this topic so I suck at explaining

 

[rama_v]

If you want to go more deeply into it, you have to look at modes of vibration within the cavity. Think of these like the standing waves on a string. The higher frequencies have more possible modes, so classically one predicted that the shorter the wavelength, the greater the number of modes, and hence the greater the intensity of the radiation. But if we introduce the caveat that energy packets (read: photons) have discrete, rather than continuous energy spectra, then, mathematically, the whole thing works, and so we can conclude that energy is quantised.

For more info, check out Planck's law - Wikipedia, the free encyclopedia . You will see the analogue to the particle in the box problem (which, if it was in the HSC, would be useful tangentially to this particular dot-point!).

This is a really interesting dotpoint, which unfortunately cannot be done justice at the HSC level. Do physics at uni if you are keen