  Complementarity is usually considered as a phenomenon of microscopic systems.
In this paper we report an experimental observation of complementarity in the
correlated double-slit interference with a pseudothermal light source. The
thermal light beam is divided into test and reference beams which are
correlated with each other. The double-slit is set in the test arm, and the
interference pattern can be observed in the intensity correlation between the
two arms. The experimental results show that the disappearance of interference
fringe depends on whether which-path information is gained through the
reference arm. The experiment therefore witnesses the complementarity occurring
in a macroscopic system.
