[hamdani yusuf (OP)]

All he needed was quantised oscillators in the walls, the quantisation of any radiation that could be found inside the cavity would follow automatically.   This has some relevance for suggesting one reason why all bits of light you might find anywhere (e.g. not just inside a cavity) would still be quantised:   If every bit of light (anywhere) has it origin ultimately in a transition that happened in an atom, then the source of all e-m radiation is suitably quantised,  so all the radiation you will find will automatically be suitably quantised.

Some e-m radiations come from molecular activities. Some others come from macroscopic mechanical vibration.

[Bored chemist]

Some e-m radiations come from molecular activities. Some others come from macroscopic mechanical vibration.

Has em radiation from a  macroscopic vibration ever been detected?

[Eternal Student]

Hi.

    The phrase "don't shoot the messenger" comes to mind.   I am not the one saying that all Planck originally assumed was quantised oscillators in the walls.   The history books are saying that. 
     However, later versions of the derivation for the blackbody spectrum will often assume that it was the radiation inside the cavity itself that was directly quantised.  So you can find many derivations in many textbooks that do directly make that assumption and they may even be better or superior derivations of the result.   We're all in agreement that we now have enough evidence to suggest that radiation is quantised (everywhere).   For example, Einstein's work on the photo-electric effect did suggest that.   However, what we now assume and may use in a modern derivation for the Blackbody spectrum does NOT change what Planck originally assumed and how he started to explain the spectrum.

       Planck assumed that the sources of radiation are atoms in a state of oscillation and that the vibrational energy of each oscillator may have any of a series of discrete values but never any value between.

[Taken from Encyclopedia Britannica:  https://www.britannica.com/science/Plancks-radiation-law]

      Planck, in his treatment to solve blackbody radiation, considered that the electromagnetic waves inside the cavity of the blackbody are standing waves due to oscillating charges on the wall of the cavity. At equilibrium, the frequency of oscillation of the oscillating charge is equal to the frequency of the electromagnetic wave produced by it i.e. at equilibrium, energy of the oscillating charge is equal to the energy of the produced electromagnetic wave.
   

https://physics.stackexchange.com/questions/703294/plancks-assumptions-for-treatment-of-blackbody-radiation

     Max Planck concentrated on modelling the oscillating charges that must exist in the oven walls, radiating heat inwards and, in thermodynamic equilibrium, themselves being driven by the radiation field. He found he could account for the observed curve if he required these oscillators not to radiate energy continuously, as the classical theory would demand, but they could only lose or gain energy in chunks, called quanta, of size hν, for an oscillator of frequency ν

https://chem.libretexts.org/Courses/Pacific_Union_College/Quantum_Chemistry/01%3A_The_Dawn_of_the_Quantum_Theory/1.02%3A_Quantum_Hypothesis_Used_for_Blackbody_Radiation_Law

    Then there's the source of information that Hamdani presented himself in post #83.  Just read that carefully and you will see it is talking about the oscillators in the walls.

This is a non-exhaustive list of information sources.  Do your own historical research and you will turn up more evidence.    It really does seem that Planck's original assumptions focused on the oscillators in the walls of the cavity.   The quantisation of the radiation that was inside the cavity just followed as a consequence of that.   Indeed, the very first derivation proposed by Planck didn't offer much explanation for the quantisation what-so-ever,  there may not have been any explanation on a microscopic scale.   He just noted that if you assume the energy was quantised then you do get a spectrum which matches with the experimental results well.   I'm not a expert on the History of science but it was shortly after that when his first attempts at providing a microscopic explanation was made - those early attempts focused on quantising the oscillators in the walls.

As stated in my earlier post: 

Yes, you can derive the blackbody spectrum by assuming radiation is quantised but you just don't have to and historically it does not look like Planck made that assumption.   All he needed was quantised oscillators in the walls, the quantisation of any radiation that could be found inside the cavity would follow automatically.

- - - - - - - - - - - - - -

Some e-m radiations come from molecular activities. Some others come from macroscopic mechanical vibration.

   As @Bored chemist  started to suggest - that is not actually obvious.   While we can model radio waves as being something that might be produced in an aerial just by charges being accelerated up and down the aerial,  it is still almost certainly just a model.   Each individual photon (or bit of radio wave) probably is being produced by some interaction(s) within atoms (or molecules with hundreds of interacting atoms) i.e. as a consequence of some fundamental atomic scale production mechanism.

Best Wishes.

[hamdani yusuf (OP)]

Some e-m radiations come from molecular activities. Some others come from macroscopic mechanical vibration.

Has em radiation from a  macroscopic vibration ever been detected?

Something like this.

[hamdani yusuf (OP)]

However, what we now assume and may use in a modern derivation for the Blackbody spectrum does NOT change what Planck originally assumed and how he started to explain the spectrum.

I wonder how can anyone disagree with this.

[Bored chemist]

Something like this.

I see no photons.

[Bored chemist]

I suspect that the reason this is such a  long thread is that the subject is this "Where does quantization of energy of electromagnetic radiation come from?".
And essentially that's asking why quantisation happens.
In general, science isn't good at saying "why" things happen.
We can put forward models of what we expect to happen; but that's not teh same thing.
I'm not sure that anyone can really explain why an apple falls from the tree.
We can say that that everything attracts everything, but we can't say why everything attracts everything.

You can make up an explanation in terms of gravitons and that's great- it even sort-of explains an inverse square law.

But it still doesn't say why gravitons exist.

[alancalverd]

I think the question is back to front. It is sensible to ask why a quantum change of energy levels produces electromagnetic radiation,and the answer is quite simply "conservation", but not every em spectrum is necessarily quantised.

[Eternal Student]

Hi.

I wonder how can anyone disagree with this.

   i.d.k.      Do some historical research,  find some document showing Planck actually proceeded in some other way initially.

      I'm not an expert in the History of science.    I've just spent a bit of time looking at Blackbody radiation because some things have worried me, specifically I couldn't see why some simplifications work as well as they did etc.   So I made some efforts to find several variations in the derivation and to try and separate what Planck did initially from the more modern re-workings of the derivations that can be found in textbooks.
   This thread:  https://www.thenakedscientists.com/forum/index.php?topic=86164.0   appeared some time ago - although the topic changed into a discussion of something else at the end.   You (Hamdani) were one of the frequent contributors to the post towards the end.   There wasn't a definitive conclusion reached and at least one aspect of the typical derivation of the Blackbody spectrum still seems unsatisfactory to me,  it seems like a bit of a fudge or cheat.   I can't get any LaTeX support on this forum so there's no easy way to communicate that bit of the derivation and it's also not especially relevant to this forum thread.

Best Wishes.

[Bored chemist]

but not every em spectrum is necessarily quantised.

OK, once you have quantised energy states then conservation explains why they gain or lose energy in quanta.

Why is any of them quantised?
Why does a hydrogen atom have quantised energy?

[alancalverd]

Why does a hydrogen atom have quantised energy?

It's all down to Heisenberg and Schrodinger. For other atoms, you also need a teaspoon of Pauli.

[Bored chemist]

Why does a hydrogen atom have quantised energy?

It's all down to Heisenberg and Schrodinger.

No.
Grass was green before either of them was born.

[hamdani yusuf (OP)]

I found some old videos explaining how Planck came up with his law.

[alancalverd]

Why does a hydrogen atom have quantised energy?

It's all down to Heisenberg and Schrodinger.

No.
Grass was green before either of them was born.

I smell a philosopher! Indeed, only God can answer the question "why", and as there is no God, the question is meaningless.  But if you want to know "how", I refer to the three hon gents I mentioned earlier.

As for the color of grass, I was blissfully unaware that hydrogen had such a strong absorption in the visible red spectrum that it makes grass look green. I thought it was something to do with the molecular orbitals of chlorophyll (Schrodinger and Pauli again - and the clue is in the name!)  Clearly my botanist brother in law has been misleading me.

[Bored chemist]

Ho hum...

I thought it was something to do with the molecular orbitals

Yes.
And we model the molecular orbitals as combinations of atomic orbitals.
And we model atomic orbitals as if they are "hydrogen like atoms".
So, if hydrogen energy levels weren't quantised, the orbitals of the other atoms wouldn't be.
And if the other atoms' orbitals were not quantised, then the orbitals of molecules wouldn't be.
And if the energy levels of molecules weren't quantised  then grass wouldn't be green.

But we know that grass was green before Heisenberg and Schrodinger were born.
So we know that quantisation is not a consequence of anything they did, said or were.

So we know that you were wrong.

Why not just accept it next time and save bandwidth?


Incidentally, hot hydrogen atoms do have a strong absorption for red light.
It's the hydrogen alpha line looked at from the other point of view.

[alancalverd]

So we all look forward to your explanation without using anything that Heisenberg, Schrodinger or Pauli wrote.

And we model the molecular orbitals as combinations of atomic orbitals.

Which would produce some very odd-shaped molecules indeed. Whatever happened to delocalisation since 1963?

[Bored chemist]

So we all look forward to your explanation without using anything that Heisenberg, Schrodinger or Pauli wrote

I already gave it.
Science can't tell you "why" something happens.

I'm seeking to explain why the thread is going nowhere and I didn't need to mention any physicists to do so.

Whatever happened to delocalisation since 1963?

I presume that among the things that happened was that you forgot how it works.
When people talk about things like pi bonds and sigma bonds they are talking about the (dominant) atomic orbitals from which they are formed.
Things like "sp² hybrid" even tell you what proportions you need to mix.

[Eternal Student]

Hi.

About the videos:
   I've only glanced at some bits of the videos you (Hamdani) presented.   They seem to support the idea that Planck focused on the oscillators in the walls.  See, for example,  around 6:10 ~ 6:30 in the first video where the narrator states that light came from those quantised oscillators. 

   I found at least one statement that was made which isn't necessarily true.   In the second video at about 7:00 to 7:30  the narrator discusses the use of a cuboidal shaped cavity for the mathematical model (which is the typical choice).   There is a fairly casual statement made that any shaped cavity could have been used, the result would be the same but the maths would just be harder.   That is not at all obvious and I don't think it should be stated as established fact.  There are several adjustments to the method that would normally be made.  In particular, there are some extra assumptions that seem essential and must be added to the mathematical model to correctly predict that very small and peculiarly shaped cavities will still ultimately produce the usual BB spectrum.
   There are some pieces of equipment which work purely because there is a cavity (including a suitable material for the walls of the cavity) with the right shape and dimensions to seriously influence the spectrum or frequency of radiation that is supported.   (See, for example, the discussion of lasers, resonance and cavities  in this post  https://www.thenakedscientists.com/forum/index.php?topic=86164.msg700593#msg700593   which you (Hamdani) seemed to approve of at the time).   
 
   The videos do seem to take a fairly conventional approach and walk you through how you must have stationary e-m waves that fit inside the cavity,   only these will be considered as supported "modes" of radiation.    Overall, the mathematical model discussed (which is a typical derivation for the BB spectrum) is just an approximation but the narrator doesn't make it apparent where all of those approximations are made.   For example, at time 8:05 to 8:30 in the second video a formula for the density of modes is said to be "not easy to find" and just written down.   This is important because it's only this approximation which allows us to finish with a continuous spectrum rather than just a discrete set of frequencies.    So, it's important to note that the entire method, even for simple cuboidal shaped cavities, is just an approximation.   As suggested earlier, you need some additional approximations or assumptions before you could be certain that an arbitrary shaped cavity would still give rise to the usual BB spectrum.

[LATE EDITING:  This post is already long, I've removed further discussion about the mathematical model(s) used for deriving the BB spectrum in very small and peculiarly shaped cavities.   There's no need to side-track the thread too much].

Best Wishes.

[Bored chemist]

 I found at least one statement that was made which isn't necessarily true.   In the second video at about 7:00 to 7:30  the narrator discusses the use of a cuboidal shaped cavity for the mathematical model (which is the typical choice).   There is a fairly casual statement made that any shaped cavity could have been used, the result would be the same but the maths would just be harder.   That is not at all obvious and I don't think it should be stated as established fact.

Imagine that you wanted to model a BB of some other shape.
You could make it out of small cube shaped BB (for which the maths is known) an then recognise that, if they are all at the same temperature, there's no net radiation exchange between the cubes; only at the surface.
And you can then rescale the problem - effectively using a different unit of length- for example, you could use cubic centimetre cubes to fill up a six foot sphere and calculate the wavelengths in fathoms.

[hamdani yusuf (OP)]

Incidentally, hot hydrogen atoms do have a strong absorption for red light.
It's the hydrogen alpha line looked at from the other point of view.

How hot or cold does it take to produce absorption spectrum?

http://www.4college.co.uk/as/el/how.gif

https://www.daviddarling.info/images/types_of_spectra.jpg


https://sites.ualberta.ca/~pogosyan/teaching/ASTRO_122/lect6/figure05-14.jpg