Max Planck introduced the concept of energy quantization in 1900.
Planck developed the idea while studying the spectrum of black-body radiation, the light emitted by an idealized object in thermal equilibrium. Classical physics could not correctly explain the observed distribution of wavelengths. To match the data, Planck proposed that oscillators in matter could exchange energy only in discrete amounts proportional to frequency: E = hf, where h is Planck’s constant.
Planck initially treated quantization as a mathematical assumption needed to solve the radiation problem, rather than as a fully established description of nature. The idea soon became foundational. In 1905, Albert Einstein argued that electromagnetic radiation itself could behave as discrete quanta when explaining the photoelectric effect.
Planck’s work helped launch quantum theory, although the modern interpretation developed gradually. Niels Bohr applied quantized energy levels to the atom in 1913, and later quantum mechanics replaced the older semiclassical models. The year 1905 is therefore associated with Einstein’s light-quantum paper, not Planck’s original introduction of energy quantization.