A European Informational Website
learn more
In modern physics, the photon is the elementary particle responsible for electromagnetic phenomena. Usually, it is considered a carrier of electromagnetic radiation such as light, radio waves, and X-rays. In the remainder of this article, the term "light" refers to all types of electromagnetic radiation. The photon differs from other elementary particles such as the electron and the quark in that it has zero mass[1] and that in vacuum, it always travels at the speed of light, c. Like all quanta, the photon has both wave and particle properties ("wave–particle duality"). As a wave, a single photon is distributed over space and shows wave-like phenomena such as refraction by a lens and destructive interference when reflected waves cancel each other out. However, as a particle, it can only interact with matter by transferring the amount of energy where h is Planck's constant, c is the speed of light, and is its wavelength. For visible light, the energy carried by a single photon would be around a tiny joules. However, this energy is sufficient to, for example, excite a single molecule in the retina of a human eye, thus contributing to human vision.