German physicist who formulated an equation describing the blackbody spectrum in 1900. Wien and Rayleigh had also developed equations, but Wien's only worked at high frequencies, and Rayleigh's only worked at low frequencies. Planck's spectrum was obtained by postulating that energy was directly proportional to frequency (). Planck believed that this quantization applied only to the absorption and emission of energy by matter, not to electromagnetic waves themselves. However, it turned out to be much more general than he could have imagined.

Planck received the Nobel Prize in physics in 1918 for his quantum theory after it had been successfully applied to the photoelectric effect by Einstein and the atom by Niels Bohr. Planck showed there were difficulties in relating the statistical theory of molecular motion to the thermodynamical approach. He also criticized the probabilistic interpretation of entropy. He was the first to write down the equation usually attributed to Boltzmann, . In fact, the constant k (as opposed to R/N_A, where R is the universal gas constant and N_A is Avogadro's number ) was first used by Planck in 1900. Lorentz and others called k Planck's constant until 1911 (Pais 1991, p. 60), when the term Boltzmann's constant became generally accepted.

Planck was also a philosopher of science. In his Scientific Autobiography and Other Papers, he stated Planck's Principle, which holds that "A new scientific truth does not triumph by convincing its opponents and making them see the light, but rather because its opponents eventually die and a new generation grows up that is familiar with it." This view contradicts that forwarded by Karl Popper known as Popper's Principle.

Additional biographies: MacTutor (St. Andrews), Bonn