# Electromagnetic field of unpolarized light

I need help in finding an expression for the instantaneous electric and magnetic field of unpolarized light in order to write down and evaluate the time-averaged norm of the Poynting vector (i.e. the intensity of unpolarized light). I expect this to be a superposition of some basis functions, but how many and how do they look like? Are these linear polarized waves with random orientation and phase shift?

$$\bar{E}(t,\bar{x})=\sum_{i}\bar{E}_{0,i}e^{i(\bar{k}\cdot\bar{x}-\omega t+\delta_{i})}$$ with $\|\bar{E}_{0,i}\|=\|\bar{E}_{0,j}\|$ ?

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possible duplicate of Intensity of unpolarized light –  Colin K Jul 13 '12 at 16:35
it is not a duplicate, only loosely related. –  Arnold Neumaier Jul 13 '12 at 17:28

@Wox: Yes, even when plotting it over a single wavelength. (But one cannot measure that finely.) It causes the superposition to be zero, but not the intensity, as the latter is quadratic in the amplitude. (Just like ordinary scalar noise averages to zero, but its square averages to a positive variance.) The link en.wikipedia.org/wiki/Degree_of_coherence may help; if you replace $E$ by a vector, you get correlation matrices, which in the stationary case describe polarization. –  Arnold Neumaier Jul 16 '12 at 11:23