he maximum kinetic energy K_max of an ejected electron is given by
K_max = h,f - varphi,
where h is the Planck constant and f is the frequency of the incident photon. The term varphi is the work function (sometimes denoted W, or phi[11]), which gives the minimum energy required to remove a delocalised electron from the surface of the metal. The work function satisfies
varphi = h,f_0,
where f_0 is the threshold frequency for the metal. The maximum kinetic energy of an ejected electron is then
K_max = h left(f - f_0
ight).
Kinetic energy is positive, so we must have f > f_0 for the photoelectric effect to occur.[12]