The form of electromagnetic radiation to which the human eye is sensitive and on which our visual awareness of the universe and its contents relies (see colour).
The finite velocity of light was suspected by many early experimenters in optics, but it was not established until 1676 when Ole Røemer (1644–1710) measured it. Sir Isaac Newton investigated the optical spectrum and used existing knowledge to establish a primarily corpuscular theory of light, in which it was regarded as a stream of particles that set up disturbances in the ‘aether’ of space. His successors adopted the corpuscles but ignored the wavelike disturbances until Thomas Young rediscovered the interference of light in 1801 and showed that a wave theory was essential to interpret this type of phenomenon. This view was accepted for most of the 19th century and it enabled James Clerk Maxwell to show that light forms part of the electromagnetic spectrum. He believed that waves of electromagnetic radiation required a special medium to travel through, and revived the name ‘luminiferous ether’ for such a medium. The Michelson–Morley experiment in 1887 showed that, if the medium existed, it could not be detected; it is now generally accepted that the ether is an unnecessary hypothesis. In 1905 Albert Einstein showed that the photoelectric effect could only be explained on the assumption that light consists of a stream of discrete photons of electromagnetic energy. This renewed conflict between the corpuscular and wave theories has gradually been resolved by the evolution of the quantum theory and wave mechanics. While it is not easy to construct a model that has both wave and particle characteristics, it is accepted, according to the theory of complementarity proposed by Niels Bohr, that in some experiments light will appear wavelike, while in others it will appear to be corpuscular. During the course of the evolution of wave mechanics it has also become evident that electrons and other elementary particles have dual wave and particle properties.