# Planck law derivation, number of modes, average energy

I am just starting learning quantum physics, and I am starting from old quantum theories and from black body radiation, but I am getting in trouble to understand Planck law derivation. I am too confused about that derivation. In my textbook it is written that Planck derived the average energy of each oscillating dipole, and my teacher says that those dipoles are actually electrons in different energy states. So he derived the average energy for each electron and multiplied it with the "possible number of standing wave modes" to get the energy distribution.

But I used to think that he wanted to calculate the energy density of radiation inside the black body, so we should calculate the avarage energy for this radiation (electromagnetic waves) or the energy which is emitted by those electrons, not the avarage energy of electrons itself.

Electrons will not radiate their whole energy (which is actually the average energy of each electrons). So we need to figure out what is the radiated energy by each electron, not their own whole energy, as they will release a little bit of energy to minimize their potential energy. Then we should multiply it by the total number of modes, which is the possible number of orientations of electromagnetic waves, or energies released by those electrons. Tell me if I am wrong in this sense.

Please help me out of this trouble , I am totally confused about those modes and average energy. I can't understand properly which average energy are they talking about.

• Your teacher is wrong. You should be counting the modes of the electromagnetic field itself. Planck's law works even in cavities where there is no matter and thus no electrons. – Raziman T V Dec 29 '16 at 6:16
• In his original paper Planck only assumed that the interaction between matter and light is quantized not the EM field itself. Only afterwards Einstein, while explaining photo-electric effect, first assumed that EM field itself is quantized. – Ari Dec 29 '16 at 6:38
• So those oscillators ,what the Planck talked about are ,photon? And the average energy is of the photon? – user101134 Dec 29 '16 at 6:46