Lec 3 Planck's Radiation Law

This lecture is part of a series on quantum mechanics. It begins by reviewing the observed behavior of blackbody radiation, including the shift of the peak wavelength with temperature (Wien’s displacement law) and the total energy dependence on temperature (Stefan-Boltzmann law). The instructor then introduces the problem of deriving the spectral energy density function. He discusses the classical attempts, particularly the Rayleigh-Jeans law, which correctly predicted low-frequency behavior but failed at high frequencies, leading to the ‘ultraviolet catastrophe’. The lecture then presents Planck’s solution, which involved the radical assumption that energy is quantized, with energy proportional to frequency (E = nhν). This assumption led to the Planck radiation law, which perfectly matched experimental data. The instructor emphasizes that Planck himself was uncomfortable with this assumption, as it lacked a classical justification. The lecture concludes by noting that this quantization marked the birth of quantum mechanics.