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Max Planck studied in Munich and Berlin under Kirchhoff and Helmholtz, and he eventually succeeded Kirchhoff's position in Berlin. First, his interest was in thermodynamics, but stimulated by Kirchhoff's work on radiation from a black body, he began to work on radiation. In the process of pursuing this problem, he seeked a general law of radiation, and in 1900 he arrived at a formula, which opened a new epoch in micro-physics.
He gave a theoretical consideration on this formula later on, and found that the light contained in the radiation under investigation has energy E distributed according to the formula
E = n(hΛ)
where n is an integral number, Λis the frequency of light and h is a small constant. Planck came to the conclusion that energy cannot take a continuous value but has a discrete value regulated by this constant. This idea, that energy jumps from a value to another value having this unit of h, is Planck's quantum hypothesis, and h is the Planck constant, which turned out to be one of the basic constants in physics.
The power of this hypothesis was amply confirmed later, by works of such people as Einstein (light quanta hypothesis for photo-electric effect) or Bohr (atomic model). And Planck was awarded the Nobel Physics Prize in 1918 for this work. But even a greater revolution was still to come in the 1920's, i.e. the birth of quantum mechanics.