When electromagnetic radiation is incident on the surface of certain metals electrons may be ejected. A photon of energy hf penetrates the material and is absorbed by an electron. If enough energy is available, the electron will be raised to the surface and ejected with some kinetic energy, ½mv2. Depending on how deep in the material they are, electrons have a range of valuesof KE will be emitted. Let φ be the energy required for an electron to break free of the surface, the so-called work function. For electrons up near the surface to begin with, an amount of energy (hf - φ) will be available and this is the maximum kinetic energy that can be imparted to any electron.
Accordingly, Einstein's photoelectric equation is

½mv2max = hf - φ

The energy of the ejected electron may be found by determining what potential difference must be applied to stop its motion; then ½mv2 = Vse. For the most energetic electron,

hf - φ = Vse

where Vsis called the stopping potential.

For any surface, the radiation must be of short enough wavelength so that the photon energy hf is large enough to eject the electron. At the threshold wavelength (or the well-known frequency), the photon's energy just equals the work function. For ordinary metals the treshhold wavelength lies in the visible or ultraviolet range. X-rays will eject photoelectrons readily; far-infrared photons will not.