Photoelectric effect
An effect described by A. Einstein that demonstrates that light appears to be built up of photons or particles. Light can do excite electrons (called photoelectrons in this context) to be ejected through a metal. Light along below frequency a sure threshold, at any particular intensity, will not cause any photoelectrons to be emitted from the metal. Above that particular frequency, photoelectrons are emitted in proportion to the intensity of incident light.
The cause is that a photon contains energy in proportion to its wavelength, & the constant of proportionality is the Planck constant. Below a firm frequency and therefore below a certain energy -- the incident photons do not have enough energy to knock the photoelectrons out of the metal. Above that threshold energy, called the work function, photons will knock on the photoelectrons out of the metal, in proportion to the number of photons (the intensity of the light). On to higher energies & frequencies, the photoelectrons ejected get a kinetic energy corresponding to the difference among the photon's energy and the workfunction.