In quantum interpretation of the electromagnetic waves in vacuum the photonhas the energy E = hw/2n and the momentump= hk/2n. So the ratioEof the energy to the momentum is= c, the speed of light inkvacuum.Similar relation can be obtained in classical electrodynamics as follows.Consider the time-averaged energy density of the electromagnetic fieldɛ = B² /2µ0 + €0 E² /2 for a plane wave propagating in vacuum along thez-direction. Calculate the time-averaged energy flux (Poynting flux) for such awave S = E × H and confirm that its ratio to the time-averaged energydensity ratio is equal to the speed of light in vacuum, S/e = c.

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