  The interference phenomenon of light is a common but most important effect in
physics. In this article, we compare and analyse the interference effects of
both coherent and incoherent light; in particular, the interference of
spatially incoherent light sources is described. Thermal light and a two-photon
entangled source may both be regarded as incoherent sources which can generate
similar second-order interference effects, but their underlying physics is
quite different. First-order interference can be realized with both spatially
coherent and incoherent sources in the same well-designed experimental setup,
but exhibit different interference patterns; their different interference
mechanisms will also be clarified.
