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.
