190 1 year, Max Planck published a research report, and his prediction of the emission spectrum of blackbody in equilibrium was completely in line with the experimental data. In this report, he made a special mathematical hypothesis to quantize the electromagnetic radiation energy emitted or absorbed by the harmonic oscillator (the atom that constitutes the surface of the black body wall). He called it discrete energy quantum.

Where h is discrete energy and Planck constant.

This is the famous Planck relationship. Based on Planck's hypothesis, Planck deduced a law of blackbody energy distribution, which is called Planck's law of blackbody radiation.

Planck's law of blackbody radiation (Planck's law or blackbody radiation law for short) is a formula used to describe the relationship between the emissivity of electromagnetic radiation emitted by blackbody and the frequency of electromagnetic radiation at any temperature. Here emissivity is a function of frequency: