In physics, light is regarded as an electromagnetic wave that propagates (in a vacuum) with the speed of light. If the wave passes through a medium, its propagation is determined by a material constant (the complex refractive index). Two variables characterize the electromagnetic wave: the wavelength, which determines the light's colour, and the intensity, which determines the light's flux, i.e. the rate of energy flow per unit of cross-sectional area. The wavelength of visible light ranges from 400 nm (violet) to 700 nm (red). Most photodetectors pick up a wider range (200..1000 nm).
Fig. 26: Spectrum of electromagnetic waves from 1..109 Å