At the end of the 19th century, classical physics was capable of explaining almost all known physical phenomena. However, phenomena such as black body radiation, the photoelectric effect, atomic spectra, or the Compton effect did not have a satisfactory explanation.

The reason why classical physics is unable to explain such phenomena is because it is not an adequate theory to describe phenomena at the atomic level. In these dimensions a new theory is required, developed during the first quarter of the 20th century, called the quantum theory.