Hi! In classical electromagnetic theory the energy of an electromagnetic wave is proportional to its amplitude squared. In contrast, the quantum mechanical equation ##E = \hbar \nu## states that the energy is proportional to the frequency of the wave (photon). Now, according to the correspondence principle, quantum physics must reduce to classical physics as the quantum numbers get sufficiently large: which in this case would amount to having a large enough number of photons. So my question is, how does one go from the quantum mechanical description ##E = \hbar \nu## to the classical description of the energy being proportional to the square of the amplitude?