 The energy of each packet is related to the angular frequency of the wave by the relation

where formula_41 is an experimentally determined quantity known as the reduced Planck's constant.If there are formula_42 photons in a box of volume formula_43, the energy (neglecting zero point energy) in the electromagnetic field is

and the energy density is

The energy of a photon can be related to classical fields through the correspondence principle which states that for a large number of photons, the quantum and classical treatments must agree.Thus, for very large formula_42, the quantum energy density must be the same as the classical energy density

The average number of photons in the box in a coherent state is then

The correspondence principle also determines the momentum and angular momentum of the photon.