In 1900, Max Planck succeeds in describing exactly the energy distribution of heat radiation by his famous radiation law. However, its interpretation in the framework of classical physics raises considerable problems.

Planck’s research about thermal radiation can be traced back to an intensive preoccupation with the problems of thermodynamics. The central tenet is the idea of entropy, which later is interpreted as a measure of disorder. His lectures on thermodynamics summarize this knowledge.