If a quantum of radiation with an energy greater than the work function Eo, and therefore a frequency greater than fo, falls on a surface an electron will be emitted with some kinetic energy and escape from the surface.
The animation shows the effect of radiation of different wavelengths falling on a metal surface. The depth of the potential well represents the work function of the metal. Free electrons in the surface will absorb energy from the incoming quantum.
On the left, low energy, long wavelength red radiation falls on the surface. The electron absorbs energy but the energy is not sufficient to allow the electron to escape from the potential well and it falls back again. In the centre the quantum of green light has more energy, the electron rises higher in the well but this is still not enough to allow it to escape.
On the right, high energy, short wavelength violet radiation falls on the surface. The energy absorbed by the electron is now sufficient to allow it to escape.