TITLE: Planck's postulate for oscillators or for light?
QUESTION [2 upvotes]: I know that Planck originally postulated that the energy of an oscillator in a black body was quantised to $E=nh\nu$ but did he know at the time that this meant the energy of light was also quantised (since all light has to be made from such oscillation)?

REPLY [2 votes]: Planck published several works on the theory of blackbody radiation based on different ideas, but generally the use of integer counting of energy he meant to be used for the energy of material oscillators. He did not believe the quantization applied to light itself - he assumed Maxwell's theory with its differential equations and derived his spectral function (Planck's law).
In contrast to widespread belief, Planck did not introduce quantization of EM field and he did not introduce photons. Other people after him did that.
https://en.wikipedia.org/wiki/Planck%27s_law#Trying_to_find_a_physical_explanation_of_the_law
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