Planck-Einstein Relation

Planck, a person, first proposed (and Einstein, another person, later applied to photons via the photoelectric effect) that

E=hf\begin{gather*} E=hf \end{gather*}

It means energy EE of a photon is Planck’s constant h=6.626×10−34Jsh=6.626\times 10^{-34}Js times the frequency of the light ff.

They are all variable so we can substitute them with what we want. Because it was experimental, we can’t really prove with math that this is true, but we proved it with experiments. This is called the Planck–Einstein relation.