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Before Einstein introduced the quantum theory of light, what was the explanation given for different intensities of light of the same frequency? How does the em theory of light explain intensity? Because intensity is amount of photons per area per second but that means you had to consider light as quanta which the em theory didn't do

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It can be defined as the Poynting vector, $\mathbf{S}=\mathbf{E}\times\mathbf{H}$, of the electromagnetic wave.

If frequency is held constant, intensity is proportional to amplitude squared.

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EM theory before quantitation was entirely considered as part of Wave Theory, where they are considered as simple ordinary waves. Now Intensity in Wave theory is given by the Square of Amplitude, in general.

You might also want to check this out Exact relationship between electric field and intensity

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