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How did Max Planck’s radiation law help begin quantum theory?

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Diagram of a tree leaf illustrating photosynthesis, with arrows showing sunlight, water, and carbon dioxide entering the leaf and sugars and oxygen being produced. A person is shown beside the tree to demonstrate the exchange of oxygen and carbon dioxide between plants and animals. — electromagnetic radiation Diagram of photosynthesis showing how water, light, and carbon dioxide are absorbed by a plant to produce oxygen, sugars, and more carbon dioxide.

Max Planck’s radiation law was instrumental in the birth of quantum theory because it resolved the "ultraviolet catastrophe," a failure of classical physics to explain the energy distribution of blackbody radiation.

Classical models, such as the Rayleigh-Jeans law, predicted that energy emission would increase infinitely as frequency increased, which contradicted experimental observations at high frequencies. To derive a formula that matched experimental data, Planck made the revolutionary assumption that matter does not absorb or emit energy continuously. Instead, he proposed that energy is exchanged in discrete packets called quanta.123

Planck demonstrated that the energy ( $E$ ) of these packets is proportional to the radiation frequency ( $ν$ ), expressed by the equation $E = h ν$ , where $h$ is Planck’s constant. By introducing this constant, Planck established that energy at the atomic scale is quantized. This departure from classical wave theory laid the foundation for quantum mechanics, fundamentally changing how physicists understand the behavior of particles and waves on an atomic level.143