Threshold frequency (v0) and kinetic energy of photoelectrons


For each metal, there is a characteristic minimum frequency – the threshold frequency (v0) – below which the photoelectric effect does not occur. Red light (v = 4.3 – 4.6 × 1014 Hz), for example, of any intensity may shine on a piece of potassium for hours but no photoelectrons will be released. But as soon as even a very weak yellow light (v = 5.1 × 5.2 × 1014 Hz) shines on potassium, the photoelectric effect is observed. The threshold frequency, v0 for potassium is 5 × 1014 Hz. For photoelectric effect to occur, the striking photon should have frequency more than that of the threshold frequency. If a photon of frequency (v) strikes a metal atom whose will be emitted only if v > v0. Since the striking photon has energy equal to hv and the minimum energy required to eject electron is hv0 (also called work function, W0) then hv – hv0 is transformed as the kinetic energy of the photoelectron (½ mev2) where me is the mass of electron and v its velocity. Since total energy has to conserved, one may write Einstein’s equation as



hv – hv0 = ½ mev2 = kinetic energy of electron