It is clear that the understanding that matter is made of atoms allows us to refine our calculations and go beyond the usual optics. Quantum mechanics solved many issues like the black body radiation, the emission lines of gases, stability of atoms, photoelectric effect, conduction and heat capacity of metals and so on...
I am looking for an optical property of matter that was not well accounted using classical mechanics. Even if the true nature of optical properties like the dielectric function of materials can be derived from the band structure of solids, the Drude model for metals and Lorentz model for dielectrics were derived before quantum mechanics and work very well for macroscopic experiments. I am wondering are there any optical properties of matter that were discovered before quantum mechanics that could not be explained using classical mechanics ? or at least that it took a long time to be understood microscopically.
