I know there are a variety of questions about this topic but I have been searching the internet for a long time and couldn't find a proper answer. The question is simple:

I saw the following formula in my professor's notes for Planck's formula:

$$I(\nu,T) = \dfrac{2\pi h \nu^3}{c^2} \dfrac{h\nu}{e^{h\nu/kT}-1} .$$

However, wherever I look I have seen it in a form such that:

$$I(\nu,T) = \dfrac{2\pi h \nu^3}{c^2} \dfrac{1}{e^{h\nu/kT}-1} .$$

Simply without a $h\nu$ term. Why? What is the difference exactly? Was my professor just make a mistake?

  • $\begingroup$ I don’t recognize the first formula. My guess is that it’s a mistake. I suggest asking your professor. $\endgroup$
    – G. Smith
    Sep 17, 2019 at 23:06
  • $\begingroup$ I will, as soon as possible. Then I will update it. $\endgroup$
    – Etg
    Sep 18, 2019 at 21:04
  • $\begingroup$ What is $I$ in the first place? Intensity of what? Power? Energy per frequency? $\endgroup$
    – Cham
    Oct 4, 2019 at 17:15

1 Answer 1


It turnes out to be a mistake by the professor. Second formula is correct.


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