In the text “Photon”, I propose the hypothesis about what the photon is and how the photon interacts with electric charge. Unlike the other three omnidirectionally propagated forces (electrostatic force, magnetic force, and gravitational force), the light is the only unidirectionally propagated force, and the photon is the elementary (minimum) unit of light. And I also suggest a new version of Double-Slit Experiment to prove my idea.