The minimum frequency of light needed to eject electrons from a metal is called the threshold frequency, ν0. find the minimum energy needed to eject electrons from a metal with a threshold frequency of 5.64 × 1014 s–1.

The energy carried by the incident light is

where h is the Planck constant and f is the frequency of the light. The threshold frequency is the frequency that corresponds to the minimum energy needed to eject the electrons from the metal, so if we substitute the threshold frequency in the formula, we get the minimum energy the light must have to eject the electrons: