Light

For physicists, the word “light” designates a group of electromagnetic waves. Light that can be perceived by the human eye (“visible light”) represents only a small part of what is called the electromagnetic spectrum, to which gamma rays also belong, as well as X rays, infrared, radio waves, and television waves. These various components of the spectrum are distinguished by their wavelengths and their energy. Electromagnetic radiation is by nature at one and the same time a matter of particles (a “particle” of light is a photon) and a matter of waveforms. Since Einstein we know that the speed at which an electromagnetic wave propagates itself in empty space is independent of the point of reference chosen; this speed is a universal physical constant known as c, whose value is 299,792,458 km/second.