Einstein, however, questions this generally accepted concept and in 1905 claims that in certain situations light behaves as if it consists of particles. He can now clarify the puzzling formula that Planck proposed a few years before for the spectral distribution of light for incandescent bodies.

Über einen die Erzeugung und Verwandlung des Lichtes betreffenden heurischen Gesichtspunkt

MPIWG Einstein Annalen Website