Wikipedia:Today's featured article/October 14, 2006

The photon is the elementary particle responsible for electromagnetic phenomena. It mediates electromagnetic interactions and is the fundamental constituent of all forms of electromagnetic radiation, that is, light. The photon has zero rest mass and, in empty space, travels at a constant speed c; in the presence of matter, it can be slowed or even absorbed, transferring energy and momentum proportional to its frequency. The modern concept of the photon was developed gradually by Albert Einstein to explain experimental observations that seemed anomalous by the classical wave model of light. The photon concept has led to many advances in experimental and theoretical physics, such as lasers, Bose–Einstein condensation, quantum field theory, and the probabilistic interpretation of quantum mechanics. (more...)