black body intensity vs wavelength graph & hertz's experiment (1 Viewer)

[killua_3]

i don't really get why the graph of the intensity vs wavelength graph for black body radiation graph goes up then goes down.

and how did hertz show that light is polarised?

thanks guys.

 

[k02033]

i am not 100% sure but...

Classical reasoning says the black body radiation emitted are EMR generated by accelerations of the constituent charges of the BB( i will refer these guys to be the oscillators in the BB), caused by thermal energy.

Upon studies of this blackbody EMR, we deduce the following features:
1. Emitted radiation consist of a continous spectrum of wavelengths from all portions of the EMR spectrum.
2. There is a wavelengths that is emitted at maximum intensity. And the value of that wavelengths is only dependent upon the temperature of the black body.
3. Total emitted intensity increases with temperature.

In trying to account for all these features mathematically using classical reasoning, we arrive at UV catastrophe.

Planck's hypothesis:

1. Energy of oscillators in the black body is quantised into quantum states, each with energy E=0hf, hf, 2hf....nhf (where f is the frequency of oscillation of the charges, and n is quantum number of the energy state)

2. Oscillators emit a photon with E=hf when the oscillator moves from the higher quantum state to a lower one. (so the energy emitted is energy lost by charge, and its magnitude is actually dependent on the its frequency of oscillation) And also it cant emit E=2hf, since quantum state shifts linearly, ie charge has to fall to quantum state E=2hf before it falls to E=hf.

3. For oscaillators oscillating with high frequency the quantum states are far apart on the energy diagram(charge has to experience a huge energy change, and statistically, this happens rarely) hence changes in quantum states are less likely to occur to oscillators with high frequency, so high frenquency photon emitted are rare (explains low energy density observed.

This explained the black body curve where as the classical theory did not.

 

[k02033]

so key point is that oscillator will emit a photon whenever it experience a quantum state change (fancy words for change in energy)
so photon emits when change in energy OCCURS and it doesnt care for what the magnitutde of the energy change actually is.
so number of photons (which is proportional to intensity) is actually dependent on how likely oscillators experience energy changes.

 

[killua_3]

what do you guys mean by oscillators?

 

[k02033]

when u heat something up, the energy that you input tends cause the atoms that makes up the object to oscillate (lattice vibrations that leads to resistances in conductors is an example) . and this causes electrons to ocillate about in the object.

 

[k02033]

i was goin to check this thing that i have written with my uni lectuers, but never got the time to, so it might not be 100% correct, i suggest you do a little research

(but what i written here worked for me in the HSC hahah...,so either i am right, or physics HSC markers doesnt understand physics, which is likely )

 

[helper]

You would be surprised what markers let go and stop k02033,even when they know what is wrong and right.

The trouble with understanding the black body curve is that it is based on probability distributions that students haven't learnt about.

 

[Ryuu-jin-jakka]

sound right to me

 

[k02033]

You would be surprised what markers let go and stop k02033,even when they know what is wrong and right.

ok, im going assume you are someone that has experience and somehow knows whats happening behind the hsc scene. since you sound like one.

first what exactly do you mean by "let go"? do u mean hsc markers knowing that a student's response to a question is wrong, will often decide to gives it merit regardless?

if this is happening, isnt hsc marking rather flawed? since this is unfair to students who has truly devoted themselves to having correct understandings

 

[helper]

ok, im going assume you are someone that has experience and somehow knows whats happening behind the hsc scene. since you sound like one.

first what exactly do you mean by "let go"? do u mean hsc markers knowing that a student's response to a question is wrong, will often decide to gives it merit regardless?

if this is happening, isnt hsc marking rather flawed? since this is unfair to students who has truly devoted themselves to having correct understandings

No what I mean is that in some questions bits of extraneous wrong information may be ignored in questions being marked to a low level. OR things which are wrong in physics are accepted because of the level of understanding required at high school level.

For instance the common truck trailer diagram used in explaining cooper pairs is physically wrong and breaks the law of conservation of energy but is accepted at HSC level.

 

[L]

once you go black, you never go back

 

[killua_3]

in the graph given by jacaranda (i don't know how to post images on here so ill just describe it...), the graph showed intensity vs wavelength. as the wavelength decrease towards zero, ie frequency increases, the intensity shoots up, hits a peak, then falls back down.

i'm given to understand that in classical physics, you would expect the graph to look like a hyperbola, which is not what the experimental values showed. but, i don't understand how quantum physics explains this dropping of intensity after the peak, as wavelength tends towards 0 implying that frequency is increasing. if i use planck's formula, e=hf, if the freuqnecy increase, wouldnt the energy and hence intensity increase as well? not drop back down...

 

[Dumbledore]

ok, im going assume you are someone that has experience and somehow knows whats happening behind the hsc scene. since you sound like one.

first what exactly do you mean by "let go"? do u mean hsc markers knowing that a student's response to a question is wrong, will often decide to gives it merit regardless?

if this is happening, isnt hsc marking rather flawed? since this is unfair to students who has truly devoted themselves to having correct understandings

i have 2 examples of this, one past hsc question: draw the magnetic field lines of a radial magnetic field.
just about no one did it right so the markers gave marks to those who drew lines that crossed in the centre, which was bad for the smarter students who knew that magnetic field lines couldn't cross
another one: how did hertz measure the wavelength of radio waves?
again just about no one knew about the interference measurement so they said that he said the speed of radio waves was c and used v = f(lambda) and got marks for it which again was disadvantageous to the students who didn't put this because they knew he measured it a different way to prove the speed of radio waves was c

 

[helper]

Dumbledore,can you be specific about which HSC questions you are talking about. I can't recall them but it is extremely strange now to have a marking scheme or guidelines change after marking has commenced. If what your saying is true, then it would have occured at the piloting stage.

A question like that is 2005 q19, where a large number of students interpreted it as whenever during the flight did Newton's Laws apply, when the question was specifically about after the last rocket burn. This misinterpretation meant the guidelines were changed to allow some description during other stages of flight.

 

[helper]

in the graph given by jacaranda (i don't know how to post images on here so ill just describe it...), the graph showed intensity vs wavelength. as the wavelength decrease towards zero, ie frequency increases, the intensity shoots up, hits a peak, then falls back down.

i'm given to understand that in classical physics, you would expect the graph to look like a hyperbola, which is not what the experimental values showed. but, i don't understand how quantum physics explains this dropping of intensity after the peak, as wavelength tends towards 0 implying that frequency is increasing. if i use planck's formula, e=hf, if the freuqnecy increase, wouldnt the energy and hence intensity increase as well? not drop back down...

The graph is not the plot of an individual photon of energy at each frequency. It is the sum of all the photons of an individual frequency. It is impossible to fill the very large frequency photons with the energy available, so they can't be released. (Simplified to try and help understanding)