A mechanical wave is a medium-conducted wave. A mechanical wave is the phenomenon of the collective movement of many particles participating in vibration at the same time. There are four kinds of classical field matter: electric fields, magnetic fields, light fields, and gravitational fields, which are independent field matter. This paper proposes that a light wave is the emission wave of light particles, and its essence is particles. A photon is a field energy particle with a phase property. This paper theoretically denies the superposition principle of single photon double-slit interference. A single photon cannot pass through the two slits at the same time, and a single photon can pass through one of the two slits in turn to achieve double-slit interference. A light wave is the cumulative phenomenon of the individual movement of a single photon particle with the phase property at different times. Furthermore, a verification experiment to check the superposition principle of single photon double-slit interference is presented. A light shield is used to block one of two slits in turn to ensure that each single photon can only pass through one of the two slits separately. Electric field waves, magnetic field waves, and gravitational field waves have the characteristics of mechanical waves, and because they must not be the emission waves of particles, it is more reasonable that they are medium-conducted waves. The mediums of electric field waves, magnetic field waves, and gravitational field waves are electric field medium quanta, magnetic field medium quanta, and gravitational field medium quanta, respectively. This paper further proposes that the eccentric spin and eccentric plane vibration of a photon and the energy fluctuation of a vacuum field are the possible causes for the phase property of a photon.