  The discovery of the Planck's relation is generally regarded as the starting
point of quantum physics. The Planck's constant h is now regarded as one of the
most important universal constants. The physical nature of h, however, has not
been well understood. It was originally suggested as a fitting constant to
explain the black-body radiation. Although Planck had proposed a theoretical
justification of h, he was never satisfied with that. To solve this outstanding
problem, we used the Maxwell theory to directly calculate the energy and
momentum of a radiation wave packet. We found the energy of the wave packet is
indeed proportional to its oscillation frequency. This allows us to derive the
value of the Planck's constant. Furthermore, we showed that the emission and
transmission of a photon follows the principle of all-or-none. The "strength"
of the wave packet can be characterized by zeta, which represents the
integrated strength of the vector potential along a transverse axis. We
reasoned that zeta should have a fixed cut-off value for all photons. Our
results suggest that a wave packet can behave like a particle. This offers a
simple explanation to the recent satellite observations that the cosmic
microwave background follows closely the black-body radiation as predicted by
the Planck's law.
