In 1900, Planck's paper on black-body radiation introduced his constant ; in 1905, Einstein's paper on the photo-electric effect introduced light-quanta (later called photons). I hasten past these developments with just a smattering of comments. Both phenomena dealt with the interaction of light and matter. Late nineteenth century physicists felt they understood light, thanks to Maxwell. They knew less about matter, and knew they knew less, but had no reason to suspect that Newton's laws didn't hold ``all the way down''. But the interaction between the light and matter lay at the research frontier.
Einstein proposed that light of frequency was composed of quanta, each with energy . This flies in the face of Maxwell's identification of light with electromagnetic waves. Moreover, Einstein's relation weds a particle concept (the energy of one photon) to a wave concept (the frequency). (Einstein regarded his hypothesis as provisional-- a guide to a more complete theory.)
© 2001 Michael Weiss