Einstein's conclusion from early experiments on the photoelectric effect is that electromagnetic radiation is composed of irreducible packets of energy, known as photons. The energy of each packet is related to the angular frequency of the wave by the relation ϵ ϵ = ℏ ℏ ω ω where ℏ ℏ is an experimentally determined quantity known as the reduced Planck constant. If there are N photons in a box of volume V , the energy in the electromagnetic field is N ℏ ℏ ω ω and the energy density is N ℏ ℏ ω ω V