<Dl> <Dd> K max = h (f − f 0). (\ displaystyle K_ (\ max) = h \ left (f - f_ (0) \ right).) </Dd> </Dl> <Dd> K max = h (f − f 0). (\ displaystyle K_ (\ max) = h \ left (f - f_ (0) \ right).) </Dd> <P> Kinetic energy is positive, so we must have f> f 0 (\ displaystyle f> f_ (0)) for the photoelectric effect to occur . </P> <P> The relation between current and applied voltage illustrates the nature of the photoelectric effect . For discussion, a light source illuminates a plate P, and another plate electrode Q collects any emitted electrons . We vary the potential between P and Q and measure the current flowing in the external circuit between the two plates . </P>

The surface of a metal plate is illuminated