# History of Planck's equation(s)

1. Mar 31, 2013

I've only got a layman understanding of this.

Why did Planck find quantization necessary and how did he come up with the equation E = hf?

2. Mar 31, 2013

### Passionflower

http://en.wikipedia.org/wiki/Planck's_law

3. Apr 1, 2013

4. Apr 1, 2013

### Passionflower

We seem to disagree on that, I think the article is excellent.

5. Apr 1, 2013

### DiracPool

Planck was trying to explain blackbody radiation, or basically the spectrum of energy that came out of an idealized heat/radiating emitting source. He found that as the energy put into the black body was raised, the peak frequency of radiation emitted increased up to a point in the ultraviolet spectrum, and then the peak frequency dropped suddenly. This was not expected in the models that existed at the time, and was hence dubbed "the ultraviolet catastrophe."

Planck found that the only way to deal with this catastrophe was to look at the energies emitted from the blackbody as coming in discrete packets, or quanta. The figure h, roughly 10^-34, multiplied by the frequency of a given wavelength of light, defined the energy, or the action, of that quanta.

In any case, that's the short and sweet of it, all from memory, so I hope I got it right. I think the real question you may be getting at is where did this cool h constant come from. The answer is nowhere special, it was found simply by experiment and not predicted by some grand theory of Planck's.

6. Apr 1, 2013

Sorry, that wasn't my question.

That's my question. Why are quanta necessary? And E = hf

7. Apr 1, 2013

### DennisN

8. Apr 1, 2013

Yeah, the photoelectric effect does explain it nicely.

But I want to know how Planck manged to come up with it by studying.. ovens. (Didn't he?)

9. Apr 1, 2013

### DiracPool

The long answer is given in the link Passionflower gave you, the short answer is that it has to do with the thermodynamics of the blackbody radiator. The energy of the radiating body is yielded through transitions of electrons in orbitals in the blackbodies atoms. These transitions occur in discrete jumps, which are parameterized by plancks constant. That parameterization yields discrete energy units, or quanta, which manifest as emitted photons of discrete energy given by E=hf.

10. Apr 1, 2013

### vanhees71

The photoelectric effect does not shed light on the quantization of light but on the quantization of the absorption of electromagnetic field energy due to the quantum nature of electrons. You get the photo effect from first-order time-dependent perturbation theory, where you treat the interaction of a qunatized bound electron with a classical electromagnetic plane wave as the perturbation.

11. Apr 1, 2013

But presumably both electrons and photons must have a quantum nature.

What is perturbation exactly?

12. Apr 1, 2013

Guess I'll have to do it the long way.

13. Apr 1, 2013

### cabrera

Plank didn't know anything about quatum nature of electron? Were photon known on that time.

14. Apr 1, 2013

??..

15. Apr 5, 2013

### capcom1983

plancks equation is also ahead of its time it only really made sense after einstiens theory of relativity. He was try to solve the ultraviolet catastrophe because high frequencies need a high quanta of energy.

16. Apr 5, 2013

### capcom1983

17. Apr 6, 2013