  The first, second and higher-order interference of classical and nonclassical
light is interpreted in quantum optical coherence theory based on Feynman\rq{}s
path integral. Compared to the existed classical optical coherence theory based
on Maxwell\rq{}s electromagnetic theory and Glauber\rq{}s quantum optical
coherence theory based on wave mechanics formulation of quantum mechanics,
quantum optical coherence theory based on Feynman\rq{}s path integral provides
a new tool to study optical coherence and has the advantage of understanding
the connection between the physics and mathematical calculations. For instance,
based on the results of transient first-order interference of two independent
light beams, it is predicted that the well-accepted electric field model for
thermal light introduced by many optical textbooks may not be accurate. The
physics of two-photon bunching of thermal light and Hong-Ou-Mandel dip of
entangled photon pairs is the same, which can be interpreted by constructive
and destructive two-photon interference, respectively. The second-order
interference of two independent light beams are introduced in detail.
Interpreting the interference of light in quantum optical coherence theory
based on Feynman\rq{}s path integral is helpful to understand the physics of
light, and may eventually lead to the answer of the question puzzling us for a
long time: what is photon?
