In the late 19th century, physicists were struggling to understand the behavior of blackbody radiation—radiation emitted by a perfect absorber at a given temperature. The problem was that the predictions of classical physics, based on the laws of thermodynamics, did not match the experimental observations.

In 1900, Max Planck published a paper that revolutionized quantum theory. He proposed that energy can only be emitted or absorbed in discrete amounts, which he called "quanta." He also showed that the energy of a quantum is proportional to the frequency of the radiation.

Planck's theory could not be explained classically and marked the first revolutionary shift in ideas that we now recognize as Quantum mechanics.

This equation, known as Planck's law, perfectly matched the experimental observations and opened the door to a new understanding of energy and matter at the atomic and subatomic scales.