Spectroscopy is Applied Quantum Mechanics. Part 2: The Quantum Revolution
The article presents several theories relative to the nature of matter. The quantum theory of German theoretical physicist Max Planck proposes that the energy of a vibrating atom was related to its frequency. Albert Einstein's quantum theory of light hypothesizes that light itself had an energy that was directly proportional to its frequency. Danish physicist Niels Bohr develops an equation that could help explain the lines that appeared in the spectrum of the hydrogen atom.
Ball, David W., "Spectroscopy is Applied Quantum Mechanics. Part 2: The Quantum Revolution" (2008). Chemistry Faculty Publications. 137.