According to Einstein, each electron is knocked out when it is hit by a single photon (1) of light, the energy of which depends on its frequency - each electron absorb one and only one photon. He argued that the reason we do not normally see the particle like nature of light is due to the large number of photons involved, just as we do not see individual pixels of ink on a printed image.
a) The dependence of the ejected electron's energy on the light's frequency rather than its intensity is a direct consequence of Planck's equation relating the energy of light to its frequency.
b) The second feature arises because the threshold for production of the electrons only occurs when the photon energy is sufficient to release an electron. Increasing the intensity of the light just means more photons. It means that the key is the frecuency - the light color. A good number of blue or violet light can knock out some electrons but a billow of red or orange photons can not knock out any electron - very low frecuency, see Radiation section.
c) Finally, the process is instantaneous because the electrons do not haveto accumulate their energy from a wave that is spread out in space. Instead, each photon delivers all its energy to an electron in a single collision. If this energy is above the necessary threshold the electron will escape.