Alford and Gold, in a paper appeared in American Journal of Physics in 196? described a didactic experiment aimed at measuring the velocity of light. The apparatus was a Michelson interferometer where the source was an arc lamp (producing white light), and the path difference was large (a few metres). The output, monitored with a photodetector, was sent to a radiofrequency spectrum analyser. The spectrum was found to have a periodic variation with periodicity .
This can be understood by remarking that if the input has a delta-function autocorrelation, the output has a pair of delta-function, and then the spectrum is sinusoidal. When detected by the quadratic detector, the low-frequency fluctuation spectrum (which is the central peak in the autocorrelation in ) has a sinusoidal shape.