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