The concept of wave-particle duality in quantum theory is difficult to grasp because it attributes particle-like properties to classical waves and wave-like properties to classical particles. There seems to be an inconsistency involved with the notion that particle-like or wave-like attributes depend on how you look at an entity. The concept comes into more clear focus with the precise language of mathematics and with an experiment or demonstration of quantum interference that involves the scattering of photons by a single slit. After describing a photon as a quon, a quantum-sized entity, a precise description of the interference of quantum probability amplitudes allows determination of the photon wavelength by classical measurements of laboratory observations.