The phenomenon by which the propagation velocity of light depends on the wavelength takes the name of light dispersion. Albert Einstein in his work provided an explanation of the photoelectric effect as an example of the application of his theory of light quanta. Young and without prejudices or academic obligations, Einstein was then employed in the Swiss Patent Office and therefore had nobody to answer to for his theories. Einstein’s derivation of Planck’s distribution law through the introduction of probability coefficients for absorption, spontaneous and stimulated emission allowed the processes to be linked via these coefficients. The introduction of Einstein’s coefficients of absorption, spontaneous and stimulated emission allowed him to propose a theory capable of explaining the optical behaviour of matter. The best confirmation of Einstein’s theory came from measurements which the American physicist Robert Andrew Millikan performed during the decade between 1916 and 1926.