Light is very well described by wave mechanics, however, from photo-induced electron-emission from metals, the electrons are not excited by the intensity of the light, but only when the light is above a certain frequency threshold.
This is strange because a high enough intensity of any light should obviously provide the energy requirement of exciting the electrons, but this is not so. This suggests that the energy is related to frequency because the electrons simply don't respond to frequencies below a threshold value.
Particulate nature of light considers light to consist of discrete packets of energy, called photons, which interact with electrons to cause the energy contained in the light to disappear and then reappear as kinetic energy of the ejected electrons plus work function.
