 For a coherent state light beam so dim that its average photon number is much less than 1, this is approximately equivalent to the quantum state of a single photon.The energy per unit volume in classical electromagnetic fields is (cgs units)

For a plane wave, converting to complex notation (and hence dividing by a factor of 2), this becomes

where the energy has been averaged over a wavelength of the wave.The fraction of energy in the x component of the plane wave (assuming linear polarization) is

with a similar expression for the y component.