light

light Electromagnetic radiation to which the human eye is sensitive. Visible light is in the wavelength range from c.400nm (violet) to 770nm (red). The speed (symbol c) at which electromagnetic radiation (including light) travels in a vacuum is 299,792,458ms⁻¹. The amount of light in an area is measured in lumens. Light exhibits typical phenomena of wave motion, such as reflection, refraction, diffraction, light polarization, and interference. Sir Isaac Newton investigated the properties of light in the 17th century, and he was the first to split white light into its component colours (spectrum) with a prism. Newton believed in a corpuscular (particle) theory of light, but the wave theory was well established by the 1820s, thanks to the work of English physicist Thomas Young and French physicist Augustin Jean Fresnel. At the beginning of the 20th century, experiments on the photoelectric effect and the work of German physicist Max Planck revived the idea that light can behave like a stream of particles. Albert Einstein stated that this stream must be photons of electromagnetic energy. Quantum theory, according to which light consists of elementary particles called photons, resolved this dilemma. When light interacts with matter, as in the photoelectric effect, energy exchanges in the form of photons and so light seems to be particles. Otherwise, it behaves as a wave. See also holography; laser; relativity