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As above. As far as I understand, the frequency of a wave is also the number of waves passing a fixed point per second.

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You seem to have a misconception. The number of photons, $n$, passing a fixed point is exactly that, and is a different entity than the frequency. By definition, frequency (in general terms) is the amount of times a cycle occurs in a given interval. In photons, we refer to frequency as the amount of times it oscillates in a second, due to its wavelike nature. Its energy is described as $$E_{photon}=hf$$

However, you must take into consideration that frequency by itself can be applied to many situations. You could measure the frequency with which waves pass a given point, and call it $f$. However, it would NOT be the same as the frequency of oscillation of the wave (as you could have several waves, in your case photons, passing through).

Basically, whenever you are calculating power you need to know the rate at which energy is provided or consumed in a given amount of time. In your case, you wish to know how much power is provided by the n photons that pass per seconds. So $f$ is the frequency of the photon that gives you the energy of said photon, and $n$ is the amount of photons that pass per seconds (or frequency of the passage of photons, but you must we careful when using frequency twice in the same sentence, as it can lead to confusion).

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  • $\begingroup$ So what is 1 photon defined as in terms of a wave? And how does this differ from a single oscillation? $\endgroup$ Jun 22, 2019 at 18:55
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    $\begingroup$ @physicsgrind You may want to post that as a separate question, as the answer is deep, nuanced, and not at all brief. $\endgroup$ Jun 22, 2019 at 19:13
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    $\begingroup$ If we have 10 crests of a wave passing a point per second, when is that not a 10 Hz wave? $\endgroup$
    – The Photon
    Jun 22, 2019 at 19:17
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    $\begingroup$ @Photon Sure. But that doesn't mean ten photons. $\endgroup$ Jun 22, 2019 at 20:18
  • $\begingroup$ @dmckee, I'm asking about the last paragraph of the answer where he says, "You could measure the frequency with which a wave passes a given point, and call it f. However, it would NOT be the same as the frequency of oscillation of the wave (not in general, at least)." As far as I can see, there's nothing about photons being discussed here. $\endgroup$
    – The Photon
    Jun 22, 2019 at 20:19

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