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Lecture demonstrations on the interference of partially coherent lighta N.I. Lobachevskii Nizhnii Novgorod State University, prosp. Gagarina 23, Nizhnii Novgorod, Russian Federation A system based on the Mach--Zender interferometer is described. It can be used for lecture demonstrations of interference between light waves with adjustable statistical parameters (degree and time of coherence). The light source is a continuously operating gas laser. Partially coherent light can be produced by modulating with one of the mirrors, using a piezoelectric barium titanate cylinder to establish the required amplitude and frequency. Another method used to vary the degree of coherence continuously from 1 to 0 relies on the introduction of a second, similar laser beam. More accurate quantitative data can be obtained by using a scanning mirror in conjunction with a photomultiplier. This transforms the intensity distribution into a ``transverse'' pattern on the screen of an oscilloscope. Polarization effects can also be demonstrated.
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