<P> If one used Planck 's energy quanta , and demanded that electromagnetic radiation at a given frequency could only transfer energy to matter in integer multiples of an energy quantum hν , then the photoelectric effect could be explained very simply . Low - frequency light only ejects low - energy electrons because each electron is excited by the absorption of a single photon . Increasing the intensity of the low - frequency light ( increasing the number of photons ) only increases the number of excited electrons , not their energy , because the energy of each photon remains low . Only by increasing the frequency of the light , and thus increasing the energy of the photons , can one eject electrons with higher energy . Thus , using Planck 's constant h to determine the energy of the photons based upon their frequency , the energy of ejected electrons should also increase linearly with frequency ; the gradient of the line being Planck 's constant . These results were not confirmed until 1915 , when Robert Andrews Millikan , who had previously determined the charge of the electron , produced experimental results in perfect accord with Einstein 's predictions . While the energy of ejected electrons reflected Planck 's constant , the existence of photons was not explicitly proven until the discovery of the photon antibunching effect , of which a modern experiment can be performed in undergraduate - level labs . This phenomenon could only be explained via photons , and not through any semi-classical theory ( which could alternatively explain the photoelectric effect ) . When Einstein received his Nobel Prize in 1921 , it was not for his more difficult and mathematically laborious special and general relativity , but for the simple , yet totally revolutionary , suggestion of quantized light . Einstein 's `` light quanta '' would not be called photons until 1925 , but even in 1905 they represented the quintessential example of wave - particle duality . Electromagnetic radiation propagates following linear wave equations , but can only be emitted or absorbed as discrete elements , thus acting as a wave and a particle simultaneously . </P>
who proposed that electrons behave like waves and particles
Einstein