Light, or electromagnetic radiation, is a transverse wave (also called a vector wave). That is, the vibrations are in a plane transverse, or perpendicular, to the direction of propagation. At the same time light is also a particle, with a `particle' of light referred to as a photon but for the purposes of monochromatic radiative transfer, only the wave properties of light need be considered. However, it will prove convenient to use the concept of photons in describing multiple scattering. The wavelike properties of electromagnetic radiation are governed by Maxwell's equations, a set of four simultaneous differential equations which connect the basic electromagnetic quantities. As is true for the complete description of any vector wave, four properties must be specified. These are: wave amplitude, wavelength, phase, and polarization.
The basic quantities associated with an electromagnetic wave are the mutually perpendicular electr