The Poynting vector is a physical magnitude that represents the local flux of electromagnetic energy. Assuming that an electromagnetic field can be described as a flux of photons, one can write

(B.13) |

where is the energy of one photon and and are the density and the speed of photons respectively. The flux of momentum can be calculated similarly. The momentum of a photon () can be obtained from the relativistic expression for the kinetic energy of one particle , where is the mass of the particle. A photon has zero mass () and the resulting expression is

(B.14) |

and the density of flux momentum can be expressed in terms of the previous relations according to

(B.15) |

Thus we have informally justified Eq. 2.16

JM Huguet 2014-02-12