The classical theory of physics, particularly according to Maxwell’s equations and the principles of thermodynamics, predicted that a black body (an idealized physical object that absorbs all incoming radiation) would emit radiation across all wavelengths. However, the predictions it made about the intensity of this radiation at different wavelengths led to what is known as the ‘ultraviolet catastrophe.’ Classical theory suggested that as the wavelength decreased, the intensity of emitted radiation would increase indefinitely, resulting in an infinite amount of energy emitted from a black body at higher frequencies.
This clear discrepancy was evidenced in experiments, where instead of continuing to increase, the intensity of radiation actually peaked at a certain wavelength and then decreased for shorter wavelengths. This contradiction between theory and observation was one of the major failings of classical physics in explaining black body radiation.
The resolution came with Max Planck’s introduction of quantum theory. In 1900, he proposed that energy is quantized and can be emitted or absorbed only in discrete amounts, or ‘quanta.’ This assumption led to Planck’s law, which accurately describes the spectral density of electromagnetic radiation emitted by a black body in thermal equilibrium. His work laid the foundation for quantum mechanics, ultimately showing that classical theories were not sufficient to explain physical phenomena at atomic and subatomic levels.