Experimental verification of the theoretical prediction by Mandel [J.
Opt. Soc. Am. 51 (1961) 1342] on spectral changes in a superposition e
xperiment has been carried out. It has been shown that when the condit
ion for interference DELTAnuT << 1 (DELTAnu is the band width of the s
pectral profile and T is the time delay between the two interfering be
ams) is satisfied, spectral modulation is observed for spectrally impu
re light. In this case the interference fringes show detailed colourin
g impressed on them.