What problem with blackbody radiation did Max Planck solve in 1900?

In 1900, German physicist Max Planck solved the problem known as the "ultraviolet catastrophe," a failure of classical physics to accurately describe blackbody radiation.

Classical theories, specifically the Rayleigh-Jeans law, predicted that the intensity of radiation emitted by a blackbody would increase infinitely as frequency increased, which contradicted experimental observations. To resolve this, Planck proposed that energy is not emitted or absorbed continuously, but rather in discrete packets called quanta.123

He theorized that the energy of these packets is directly proportional to the frequency of the radiation, expressed by the equation $E=hf$, where $h$ is Planck’s constant. By introducing this quantization, Planck formulated Planck’s radiation law, which matched experimental data perfectly and laid the foundation for quantum mechanics.124