When a particular class of partially coherent light is incident upon a circular aperture, the on-axis spectrum of the light in the far zone is different from the spectrum of the light at the aperture. An explicit expression describing the on-axis spectrum of the light in the far zone is obtained and the numerical calculation is performed. It is shown that by measuring the spectral shifts against some parameter, such as a/L~(a is the radius of the aperture and L~ is the effective correlation length of the light at the aperture at the central frequency of the source spectrum); we find that the spectral shifts show a gradual change. However when a/L~ is equal to particular values, the spectral shifts exhibit a rapid transition, i.e., spectral switches occur. Some parameters, such as the half-width at half-maximum of the source spectrum /spl Gamma/ and the ratio of the central obstruction of the source /spl epsiv//sub s/ affect the spectral switch positions and the spectral switch performance. The generation of multi-spectral lines in the far zone from a single spectral line is also shown.