The

*ansatz* encountered in so many undergraduate text books - "

*the light field can be described by *E=E

_{0}cos(

wt-kz)" - is clearly an unsatisfactory starting point for the treatment of real fields from real sources; but we usually reassure ourselves with the thought that we can surely superpose ensembles of such primitive representations by means of a Fourier integral. But can we, really?

The earliest source I've encountered for a clear and purposeful statement that it is not satisfactory to construct a theory of optical processes in terms of elementary components whose amplitudes, wavelengths and phases are all unobservable was a paper published in Madagascar in (I think) 1934; the author's aim was to develop a description of light-fields in which the unobservable oscillations did not figure.

Whether Wolf knew of this paper I don't know, but by the mid 1950s he was able to claim that

*the theory of partial coherence ... operates with quantities (namely with correlation functions and time-averaged intensities) that may in principle be obtained from experiment. This is in contrast with the elementary optical wave theory, where the basic quantity is not measurable because of the very great rapidity of optical vibrations.*

And from a different standpoint, which is not that of a book on classical physical optics such as Born and Wolf wrote, one can add that quantum theory tells us that we could

*not* know the phase of the light field of ‘the elementary optical wave theory’ if we knew its intensity, whereas the observable functions of coherence theory don't run foul of quantum theoretic inhibitions.