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This question already has an answer here:

Suppose I have a em wave train.How do I know how many photon does it contain?And where are the photons?(like in the middle or more toward the end of the wave train)The frequency of light that we get from bulbs or sun can be measured so we can accurately state its energy .In this case where does the uncertainty in time ?Moreover when it hits the detector we know about its position so how it causes the uncertainty in momentum?

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marked as duplicate by sammy gerbil, Jon Custer, Aaron Stevens, John Rennie, user191954 Oct 6 '18 at 4:41

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  • $\begingroup$ Good questions but around here you want get too many answers that may inadvertently acknowledge the idea that photons could actually be real and individual particles. Everyone talks about duality but seam to only consider waves. Funny thing is particles can be described, even to answer your questions above but waves cannot be described without incorporating billions of individual photons. $\endgroup$ – Bill Alsept Oct 5 '18 at 15:32
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    $\begingroup$ You should consider splitting this into separate questions. $\endgroup$ – user191954 Oct 5 '18 at 15:49
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    $\begingroup$ Possible duplicate of What is the relation between electromagnetic wave and photon? $\endgroup$ – sammy gerbil Oct 5 '18 at 16:03
  • $\begingroup$ 1. Spin (or helicity, actually) of photons define the polarization (of photon, and the resulting EM wave). 2. $E = n \hbar \omega$ where $n$ is the number of photons and $\omega$ is frequency. I think it would be useful to refer to these: physics.stackexchange.com/q/154468/133418, physics.stackexchange.com/q/397451/133418 $\endgroup$ – Avantgarde Oct 5 '18 at 21:09
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How do I know how many photon does it contain?

The answer could be very different depending on the source of light. Unless you have a source of pure-state light, like single-photon emitter, the question, IMHO, makes no sense.

You can ask how many photons on average do you have, i.e. what is the expectation value of the photon number operator.

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