Potential well and Photoelectric Effect

The following three diagrams show the effects of shining light of different colours (wavelengths) onto a photosensitive surface. The low energy photons of red light are unable to eject photoelectrons from the surface. The medium energy photons of green light can raise the electron to the surface of the metal, but not give it enough energy to escape. However the high energy photons of violet light give the electron sufficient energy for it to escape from the surface with some remaining kinetic energy.