  The wave-particle duality has been said to contain the entire mystery of
quantum mechanics. Many delayed-choice experiments have been performed to
further understand the wave-particle duality. Here, we reveal some flaws in the
known interference theories by comparing the theoretical predictions with the
experimental facts and show that the presence of media is a necessary condition
for interference of single photons, a photon interferes with other photon via
microscopic particles in the interface of media, and the wave-like property of
a photon originates from interaction between the photon and media; the particle
property is the inherent characteristic of a photon, and the wave-like behavior
of a photon always accompanies the particle behavior; the coherent time of
single photons cannot be longer than the lifetime of the polarized states of
the microscopic particles. Interference of single photons is interpreted
properly, and the conundrum of the wave-particle duality is well solved.
