In 1923, Millikan was
awarded the Nobel Prize in physics for his work on the elementary
charge of electricity and on the photoelectric effect We can summarise the photon picture of electromagnetic  radiation
as follows:
(i)
In interaction of radiation with matter, radiation behaves as if it is
made up of particles called photons (ii) Each photon has energy E (=hn) and momentum p (= h n/c), and
speed c, the speed of light (iii) All photons of light of a particular frequency  n, or wavelength l, have
the same energy E (=hn = hc/l) and momentum  p (= hn/c= h/l),
whatever the intensity of radiation may be