I'm studying atomic spectras and got puzzled about light-quantization. I'll expose my effort to understand it so far.

Blackbody radiation

Around the year $1900$ Planck explained blackbody radiation including a term of the form $A=k\,\nu$ which is reasonable because of the experimental results: radiating energy had been shown to decrease with frequency. If I'm not mistaken $k$ was a constant obtained by fitting the experimental data (then called Planck's constant $h$).

A couple of years later it comes the photoelectric effect, which was explained just using Planck's idea and little more. Everyone knows the formula so I won't paste it here.


Does photoelectric effect suggest that light can be thought as particle? How?

I understand perfectly $E=h\,\nu$ and that atoms have quantized energy levels, but is light energy quantized?

  • $\begingroup$ The linked article on the photoelectric effect explains it in terms of photons. Doesn't that make it clear that light is quantized? $\endgroup$
    – PM 2Ring
    Commented Jun 28, 2018 at 7:51
  • $\begingroup$ It suggests quantization, but the effect can be described as well without it, see physics.stackexchange.com/questions/68147/… $\endgroup$
    – Jasper
    Commented Jun 28, 2018 at 8:18

1 Answer 1


The formula $E=h\nu$ implies that light is quantised. Planck introduced the quantum in 1900 as a heuristic trick Then Einstein's 1905 explanation of the photoelectric effect proved that the light quantum was physical.

The reason is that the number of electrons emitted is proportional to the energy of the irradiating light divided by $h\nu$. The electron kinetic energy however increases with frequency. This means that each electron is emitted by a discrete process in which a single energy quantum $h\nu$ is absorbed.

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    $\begingroup$ Einstein didn't prove quantization. He said that the effect can easily be explained if one assumes quantization. $\endgroup$
    – Jasper
    Commented Jun 28, 2018 at 9:15
  • $\begingroup$ And the formula doesn't implies quantization, it is just what Planck need to assume, he never talked about quantization of light... $\endgroup$
    – user153036
    Commented Jun 28, 2018 at 18:08
  • $\begingroup$ @Jasper I have a different point of view and so did the physical community of his days, which awarded him a Nobel prize for this work. $\endgroup$
    – my2cts
    Commented Jun 28, 2018 at 18:25
  • $\begingroup$ @santimarandap The formula certainly implies quantisation. I did not say that Planck ever "talked about quantization of light". For such an assertion I would have to know a lot about him. To assert that he never did requires even that you know absolutely everything he ever said. Do you ? $\endgroup$
    – my2cts
    Commented Jun 28, 2018 at 18:29
  • $\begingroup$ No because I don't need to. When he published that formula, he didn't...and I don't see why it implies quantization. Does $f=ma$ for constant mass implies quantization of force? I don't see your point $\endgroup$
    – user153036
    Commented Jun 29, 2018 at 5:27

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