(color online) Explanation of why Planck’s original model works. The black-body radiation involves three steps: emission, transmission and absorption. The emitter gives out radiation energy in individual wave packets. We show that each wave packet contains an amount of energy . The emission of the wave packet is all-or-none. If the energy transition within the emitter is smaller than , no radiation wave could be emitted. When the energy transition reaches , it gives out one photon. Thus, even though the energy distribution of the emitter is continuous (solid line in the left panel), its emission of photon follows a step-wise pattern (dash lines in the left panel). When a photon is emitted, it travels as a wave packet in an all-or-none fashion. Finally, it is totally absorbed by a detector. Hence, from the point of view of detector, the energy distribution appears to follow a step-wise pattern (solid line in the right panel).