Quantum mechanics week 1 (random): Planck fortunate guess and approximations

Something interesting in the formulation of Planck’s law is that its mathematical form was obtained before considering the quantisation hypothesis. In many sources, it is said that this is Planck’s fortunate or lucky guess. At that time, classical efforts gave two approximations for the black body radiation law, one in the high-frequency regime (Wien) and one in the low frequency regime (Rayleigh-Jeans).