The graph of black body radiation (1 Viewer)

[ThreeOne]

What's the unit for the y-axis in a graph of Black Body Radiation?

 

[alcalder]

I have seen many graphs that have different labels on the Y-axis:

- radiance
- energy radiated
- intensity

but all the graphs have the same basic shape.

Wikipedia's graph is in intensity.

 

[xiao1985]

Here's a question for you alcalder:
(which troubles me greatly, because I never seem to completely understand)

According to classical understanding of light, why would intensity increase infinitely as wavelength gets shorter?!

:/

 

[zagga20]

Do correct me if im wrong but.
Classical understanding of waves:

Smaller the wavelength, the greater the frequency

the greater the frequency, the greater the energy the wave is carrying

if wavelength is approaching 0 then frequency is approaching infinity.

.'. energy would be infinite

which is impossible >_>;

 

[zingerburger]

That would be right.

 

[kony]

Do correct me if im wrong but.
Classical understanding of waves:

Smaller the wavelength, the greater the frequency

the greater the frequency, the greater the energy the wave is carrying

if wavelength is approaching 0 then frequency is approaching infinity.

.'. energy would be infinite

which is impossible >_>;

yes but that is the same with quantum physics, i.e. frequency determines the energy, from E = hf.

I think the most important thing to say here is that because energy is quantised in quantum physics, intensity is irrelevant to frequency/wavelength. hence the graph is not a simple proportionality function.

 

[xiao1985]

ok, why would classical physics say there are irradiation at low wavelength at all?

 

[MaccaFacta]

ok, why would classical physics say there are irradiation at low wavelength at all?

A bit of history might be helpful here: back in the 1800's a Scottish guy by the name of James Clerk Maxwell came up with a theory of electromagnetism which was so successful that pretty much everyone thought it was going to be the last word. For example, he predicted that electromagnetic radiation would travel at 3 x 10^8 m/s. It was only then that physicists realised that light was a form of electromagnetic radiation (EMR).

According to Maxwell's equations, when an electrically charged particle moves in a circle e.g. the in the Year 9 model of the atom, with electrons "orbiting" the nucleus, then the frequency of the emitted EMR is just the frequency at which the electron orbits e.g. if the electron orbits 50 times per second, then the EMR has a frequency of 50 Hz.

So, when Rutherford came along and showed that the nucleus was this incredibly tiny, dense, highly positively charged thing, & Maxwell's equations said that electrons orbiting the nucleus should emit EMR, "Houston, we have a problem". Of course, as soon as electrons emit EMR they're losing a bit of energy, and so they spiral in to an orbit closer to the nucleus. Because of the inverse square law for the force of attraction between electric charges, this meant that the electron started moving a lot faster, so the frequency with which it orbited the nucleus increased, and it gives off more EMR (with higher & higher frequencies - therefore shorter and shorter wavelengths). Until finally the electron collides with the nucleus. ZANG! End of atom. According to Maxwell's theory, atoms should only last for about 10^-18 seconds. Given that atoms have been around for about 4 x 10^+17 seconds, this created a slight problem for theoretical physicists. It is never good when your theory predicts something that is a million, million, million, million, million, million times smaller than an observed value.

Quantum mechanics came to the rescue. It started with Bohr and then guys like Schrodinger and Heissenberg and (eventually) Dirac gave us a decent theory about electrons that actually matched with what people measured about them in the physics lab.

Hope this helps.

 

[xiao1985]

re: above, yes it makes perfect sense...

however, this explanation can be offered with relation of maxwell's equation and rutherford's model. It does not have to mention blackbody radiation at all?

additionally, it is also a limitation in bhor's model of atom is it not? that the model cannot explain the absense of emission of EMR, despite the assumption of the model that electrons orbits the nuclei ... (not that this paragraph has any relevance to blackbody radiation)

 

[bento]

Oh my gosh you guys are crazy! You just made me feel less confident about my physics trial on friday lol...I'm pretty sure you guys are going BEYOND the scope of the syllabus..I'm not the best at physics but I don't think we need to know all of that. We only need to know Hertz's radio wave experiment, and Planck and Einstein's contribution to quantum physics and black body radiation...so basically, the stuff that we still believe today...not the history.

Either you guys go to one of those crazy schools that does way too much work for what is necessary, or you're just talking about this because you find it interesting lol...If I'm wrong them I am screwed!

 

[alcalder]

Oh my gosh you guys are crazy! You just made me feel less confident about my physics trial on friday lol...I'm pretty sure you guys are going BEYOND the scope of the syllabus..I'm not the best at physics but I don't think we need to know all of that. We only need to know Hertz's radio wave experiment, and Planck and Einstein's contribution to quantum physics and black body radiation...so basically, the stuff that we still believe today...not the history.

Either you guys go to one of those crazy schools that does way too much work for what is necessary, or you're just talking about this because you find it interesting lol...If I'm wrong them I am screwed!

Bento, don't freak. Yes, they are WAY beyond the syllabus and discussing Physics matters probably because it interests them.

Good luck in your trial. But remember, learning is not just about doing what the syllabus says - nothing new would ever be gained or ventured. Learning is about posturing and hypothesising and talking outside the realms of what you know to discover new things.