Max Planck introduced energy quanta in a 1900 paper on black-body radiation. He proposed that electromagnetic energy could be exchanged only in discrete amounts, later called quanta.
Planck’s idea arose while solving the black-body radiation problem: classical physics predicted that hot objects should emit unlimited energy at short wavelengths, a result known as the ultraviolet catastrophe. Planck’s quantization assumption produced the correct observed spectrum.
Planck initially treated quantization as a mathematical device rather than a radical description of nature. Albert Einstein later gave the idea a stronger physical interpretation in his 1905 explanation of the photoelectric effect. The constant associated with Planck’s quantum relation is now called the Planck constant.
The 1900 proposal is often regarded as the starting point of quantum theory, although quantum mechanics in its modern form developed during the 1920s through work by Heisenberg, Schrödinger, Born, Dirac, and others.